configure: force o32 ABI on MIPS

configure: force o32 ABI on MIPS

GRUB code expects O32 or N32. N32 is less tested than O32, so we prefer to
compile with O32. Some systems (e.g. GNU Guix) default to using newer
n64 or n32 ABI. Try to find suitable options to force o32.

For GCC this is simply -mabi=32. While clang supports this option as well,
o32 ABI is valid for MIPS target and n32/64 ABI are valid for MIPS64 target
only, so use "-target mips/mipsel -mabi=32".

Reported-By: Mark H Weaver <mhw@netris.org>
Also-By: Mark H Weaver <mhw@netris.org>