Merge #21320: build: fix libnatpmp macos cross compile

bd49ac416881ede3e5132789defd3fec4c6f8685 build: fix libnatpmp macos cross compile (fanquake)

Pull request description:

  Currently, our cross-compile of libnatpmp for macOS doesn't work at all.
  The wrong archiver is used, which produces an archive the linker doesn't like.
  This becomes clear when configuring:
  ```bash
  configure:25722: checking for initnatpmp in -lnatpmp
  configure:25747: env -u C_INCLUDE_PATH -u CPLUS_INCLUDE_PATH -u OBJC_INCLUDE_PATH -u OBJCPLUS_INCLUDE_PATH -u CPATH -u LIBRARY_PATH /home/ubuntu/bitcoin/depends/x86_64-apple-darwin18/native/bin/clang++ --target=x86_64-apple-darwin18 <trim>  -Wl,-headerpad_max_install_names -Wl,-dead_strip -Wl,-dead_strip_dylibs conftest.cpp -lnatpmp   >&5
  ld: archive has no table of contents for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  ```

  Fix this by using the right `ar` (we do the same for upnp).

  While we're at it, fix the build so that we are using our c/ppflags.
  In practice this basically means building with `-O2` rather than `-Os`.

  Note that this fixes an issue that is also fixed by #21209. However, given there are reservations about updating to use a newer libnatpmp source, we should just fix this for now.

ACKs for top commit:
  hebasto:
    ACK bd49ac416881ede3e5132789defd3fec4c6f8685, tested:

Tree-SHA512: 2efc2c788ef3ebebfbf564ef07b6cf63a72d8a0bccc22b0ba36537216aa575436b7e87088477e85f6d9191ad34f0b13f1c22cf88c90e1cb81641bfee5dc3058a

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