[libc] Add the get and set sys/xattr.h entrypoints. (#224729)

This is a follow-up to #224357, implementing more of sys/xattr.h. As
before, these are non-POSIX APIs.

The list of entrypoints is: `fgetxattr`, `getxattr`, `lgetxattr`,
`fsetxattr`, `setxattr`, and `lsetxattr`.

Assisted-by: Automated tooling, human reviewed.
GitOrigin-RevId: e27c7439ab588237a4c526feb3bb632f9aea8530
44 files changed