build: fix libnatpmp macos cross compile

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, we fixe the build so that we are using our c/ppflags.
This  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.

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