tools/nolibc: move MIPS ABI validation into arch-mips.h

When installing nolibc to a sysroot arch.h is not used so its ABI check
is bypassed. This makes is possible to compile nolibc with a non O32 ABI
which may build but can not run.

Move the check into arch-mips.h so it will always be evaluated.

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