symbols-check: fix `nm` invocation on MacOS

symbols-check: fix `nm` invocation on MacOS

According to Mac OSX's man page [1], 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.

[1] https://www.unix.com/man-page/osx/1/nm/

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Tested-by: Vinson Lee <vlee@freedesktop.org>