Use portable flag with nm in extract_symbols.py

Summary:
nm is one of the tools that extract_symbols.py can use to extract
symbols from llvm libraries as part of the build process. This patch
updates the invocation of nm to use the -P POSIX option for "portable
output" so we get a consistently parsable output format on all
platforms.

A link to the relevant nm format: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/nm.html

Reviewers: hubert.reinterpretcast, stevewan, sfertile

Reviewed By: stevewan

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69004