Make setting mips endian and ABI less verbose.

Allow ABI to be over ridden to allow (with other changes) programs to be
built targeting ABIs other than the default. This is used in CheriBSD.

Reviewed by: imp
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21001