MIPS: Remove -U__unix and -U__unix__ from cflags

Leave '__unix' and '__unix__' defined on mips/mips64, like arm.
This is needed to avoid a warning in external/libcxx project.

Test: successful build and boot aosp_mips-eng
Test: successful build and boot aosp_mips64-eng

Change-Id: I8f0e17bdbeffd5078c19aa7506564b8e79a9c945