tools/nolibc: add getuid() and geteuid()

This can be useful to avoid attempting some privileged operations,
starting from the nolibc-test tool that gets two failures when not
privileged.

We call getuid32() and geteuid32() when they are defined, and fall
back to getuid() and geteuid() otherwise.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>