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

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.

Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D10846