Import Clang's r280672 into our devel/llvm39 port

This revision is what's needed to make Clang work for CloudABI on ARMv6.
By pulling in this change, the ARMv6 cross compiler set up by the
cloudabi-toolchain port seems functional.

In the process, I did have to replace some ${RMDIR}s by ${RM} -rf, as it
tried to delete nonexistent directories. Brooks, do you know what's up
with that?

Submitted by: ed
Differential Revision: https://reviews.freebsd.org/D8078