[MIPSR6] R6 target options for clang

Pass along new R6 target arch options and floating
point register model options to clang.
Also pass along older arch variants.

This patch depends on recent Mips extensions to 3.5 clang.
The new options are rejected by aosp's current 3.5 clang.
This only affects builds for mips32r6/mips64r6, not Android's
default builds for mips32r2.

Change-Id: Ic921dc14ced34a83143a82e322124b3ef035014a