[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