Merge #19525: build: add -Wl,-z,separate-code to hardening flags

65d0f1a53354fb25c8152ee5b430cf57e6508594 devtools: Add security check for separate_code (Wladimir J. van der Laan)
2e9e6377f10ab1ca5021f6ec964d22161469ba60 build: add -Wl,-z,separate-code to hardening flags (fanquake)

Pull request description:

  TLDR: We are generally explicit about the hardening related flags we use,
  rather than letting the distro / toolchain decide via their defaults. This PR
  adds `-z,separate-code` which has been enabled by default for Linux targets
   since binutils 2.31. Ubuntu Bionic (currently used for gitian) ships with
  binutils 2.30, so this will enable the option for those builds.

  This flag was added to binutils/ld in the 2.30 release,
  see commit c11c786f0b45617bb8807ab6a57220d5ff50e414:

  > The new "-z separate-code" option will generate separate code LOAD
  segment which must be in wholly disjoint pages from any other data.

  It was made the default for Linux/x86 targets in the 2.31 release, see commit
  f6aec96dce1ddbd8961a3aa8a2925db2021719bb:

  > This patch adds --enable-separate-code to ld configure to turn on
  -z separate-code by default and enables it by default for Linux/x86.
  This avoids mixing code pages with data to improve cache performance
  as well as security.

  > To reduce x86-64 executable and shared object sizes, the maximum page
  size is reduced from 2MB to 4KB when -z separate-code is turned on by
  default.  Note: -z max-page-size= can be used to set the maximum page
  size.

  > We compared SPEC CPU 2017 performance before and after this change on
  Skylake server.  There are no any significant performance changes.
  Everything is mostly below +/-1%.

  Support was also added to LLVMs lld: https://reviews.llvm.org/D64903, however
  there it remains off by default.

  There were concerns about an increase in binary size, however in our case, the
  difference would seem negligible, given we are shipping a
  multi-megabyte binary, which then downloads 100's of GBs of data.

  Also note that most recent versions of distros are shipping a new enough version
  of binutils that this is available and/or already on by default (assuming the distro
  has not turned it off, I haven't checked everywhere):

  CentOS 8: 2.30
  Debian Buster 2.31.1
  Fedora 29: 2.31.1
  FreeBSD: 2.33
  GNU Guix: 2.33 / 2.34
  Ubuntu 18.04: 2.30

  Related threads / discussion:
  https://bugzilla.redhat.com/show_bug.cgi?id=1623218

  The ELF header when building on Debian Buster (where it's already enabled by default in binutils):
  ```bash
  Program Header:
      PHDR off    0x0000000000000040 vaddr 0x0000000000000040 paddr 0x0000000000000040 align 2**3
           filesz 0x00000000000002a0 memsz 0x00000000000002a0 flags r--
    INTERP off    0x00000000000002e0 vaddr 0x00000000000002e0 paddr 0x00000000000002e0 align 2**0
           filesz 0x000000000000001c memsz 0x000000000000001c flags r--
      LOAD off    0x0000000000000000 vaddr 0x0000000000000000 paddr 0x0000000000000000 align 2**12
           filesz 0x0000000000038f10 memsz 0x0000000000038f10 flags r--
      LOAD off    0x0000000000039000 vaddr 0x0000000000039000 paddr 0x0000000000039000 align 2**12
           filesz 0x00000000006b9389 memsz 0x00000000006b9389 flags r-x
      LOAD off    0x00000000006f3000 vaddr 0x00000000006f3000 paddr 0x00000000006f3000 align 2**12
           filesz 0x0000000000204847 memsz 0x0000000000204847 flags r--
      LOAD off    0x00000000008f7920 vaddr 0x00000000008f8920 paddr 0x00000000008f8920 align 2**12
           filesz 0x00000000000183e0 memsz 0x0000000000022fd0 flags rw-
   DYNAMIC off    0x000000000090adb0 vaddr 0x000000000090bdb0 paddr 0x000000000090bdb0 align 2**3
           filesz 0x0000000000000240 memsz 0x0000000000000240 flags rw-
  ```
   vs when opting out using `-Wl,-z,noseparate-code`:
  ```bash
  Program Header:
      PHDR off    0x0000000000000040 vaddr 0x0000000000000040 paddr 0x0000000000000040 align 2**3
           filesz 0x0000000000000230 memsz 0x0000000000000230 flags r--
    INTERP off    0x0000000000000270 vaddr 0x0000000000000270 paddr 0x0000000000000270 align 2**0
           filesz 0x000000000000001c memsz 0x000000000000001c flags r--
      LOAD off    0x0000000000000000 vaddr 0x0000000000000000 paddr 0x0000000000000000 align 2**12
           filesz 0x00000000008f6a87 memsz 0x00000000008f6a87 flags r-x
      LOAD off    0x00000000008f7920 vaddr 0x00000000008f8920 paddr 0x00000000008f8920 align 2**12
           filesz 0x00000000000183e0 memsz 0x0000000000022fd0 flags rw-
   DYNAMIC off    0x000000000090adb0 vaddr 0x000000000090bdb0 paddr 0x000000000090bdb0 align 2**3
           filesz 0x0000000000000240 memsz 0x0000000000000240 flags rw-
  ```

ACKs for top commit:
  laanwj:
    ACK 65d0f1a53354fb25c8152ee5b430cf57e6508594

Tree-SHA512: 6e40e434efea8a8e39f6cb244dfd16aaa5a9db5a2ea762a05d1727357b20e33b7e47c1a652ee88490c9d7952a4caa2f992396fb30346239300d37ae123e36d49

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