selftests/nolibc: simplify call to ioperm

Since commit 53fcfafa ("tools/nolibc/unistd: add syscall()") nolibc
has support for syscall(2).
Use it to get rid of some ifdef-ery.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>