selftests/nolibc: make sure gcc always use little endian on MIPS

The test on MIPS stopped working after I upgraded some of my toolchains
to use the ones from kernel.org because the mips toolchain defaults to
big endian, even though it supports both endians. Let's just add an
explicit -EL to make sure it always succeeds like the kernel does.

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