[3.11] gh-101857: Allow xattr detection on musl libc (GH-101858) (#101894)

gh-101857: Allow xattr detection on musl libc (GH-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

(cherry picked from commit 8be8101bca34b60481ec3d7ecaea4a3379fb7dbb)

Co-authored-by: Sam James <sam@gentoo.org>
Co-authored-by: Gregory P. Smith <greg@krypto.org>