gh-101857: Allow xattr detection on musl libc (#101858)

Previously, we checked exclusively for `__GLIBC__` (AND'd with some other
conditions). Checking for `__linux__` instead should be fine.

This fixes using e.g. `os.listxattr()` on systems using musl libc.

Bug: https://bugs.gentoo.org/894130

Co-authored-by: Gregory P. Smith <greg@krypto.org>