Remove LLVM build glue for MIPS as a native architecture.

[jhb]

Summary

This does not remove LLVM_TARGET_MIPS.

[emaste]

my wip branch has a more extensive change, looking... https://github.com/emaste/freebsd/commit/38e49c93e927d591325cdbe843a38dd01c7db11f

[emaste]

ah, I had also removed target support, but agree we can postpone that.
I did not have the hf/sf part in my change, probably worth calling out specifically in the commit message why hf is now not needed

[jhb]

I will include a note about hf in the commit log.