Update MIPS kernel builds to work with mips-gcc.

• Use a default -march of mips64 on N64 and N32 kernels.
• Set the endianness (via MIPS_ENDIAN) and ABI (via MIPS_ABI) in CFLAGS from MACHINE_ARCH. ARCH_FLAGS now only sets a different -march value if needed.
• TRAMP_ARCH_FLAGS inherits MIPS_ENDIAN from MACHINE_ARCH but does not set the ABI since XLPN32 needs an N64 ABI for the trampoline loader. When TRAMP_ARCH_FLAGS is used it must set both -march and -mabi.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D22030