Set correct lib32 MACHINE_ARCH for mips64el*.

Use "mipsel" instead of "mips" as the 32-bit MACHINE_ARCH when
building lib32 for little-endian 64-bit MIPS targets. This fixes an
error where some objects were compiled as LE and others compiled as BE
causing a link error for rtld32.

Reviewed by: emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23028