symbols-check: fix `nm` invocation on MacOS
According to Mac OSX's man page, this is how we should get the list of exported symbols:
nm -g -P foo.dylib
-g
to only show the exported symbols
-P
to show it in a "portable" format, ie. readable by a script
Since this is supported by GNU nm
as well, let's use that everywhere,
although some care needs to be taken as there are some differences in
the output.
Fixes the Mac OS build tests (ninja test
)