For arm targets, place ABI at the end of the target triple

[dim]

Summary

For some reason, we have been inserting the ABI specification into the
middle of the target triple, when building LLVM, like so:

armv6-gnueabi-freebsd12.0

This is the wrong way around. LLVM even auto-canonicalizes it to:

armv6--freebsd12.0-gnueabi

Let's do this the right way in llvm.build.mk instead. While here,
define a proper VENDOR macro which can be overridden easily.

[emaste]

This seems correct to me, although I'm not sure what unanticipated fallout there might be and perhaps others will comment.