build: add armhf/aarch64 gitian builds

- create a script to handle split debug. This will also eventually need to check
  targets, and use dsymutil for osx.
- update config.guess/config.sub for bdb for aarch64.
- temporarily disable symbol checks for arm/aarch64
- quit renaming to linux32/linux64 and use the host directly

This also adds a hack to work around an Ubuntu bug in the gcc-multilib package:
https://bugs.launchpad.net/ubuntu/+source/gcc-defaults-armhf-cross/+bug/1347820

The problem is that gcc-multilib conflicts with the aarch toolchain.
gcc-multilib installs a symlink that points
/usr/include/asm -> /usr/include/x86_64-linux-gnu/asm.

Without this link, gcc -m32 can't find asm/errno.h (and others), since
/usr/include/x86_64-linux-gnu isn't in its default include path. But
/usr/include/i386-linux-gnu is (though it doesn't exist on disk).

So work around the problem by linking
/usr/include/i386-linux-gnu/asm -> /usr/include/x86_64-linux-gnu/asm.

The symlink fix is actually quite reasonable, but echoing the password into
sudo is nasty, and should probably be addressed in gitian itself. It makes more
sense to enable passwordless sudo for the build user by default.

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