[runtimes] Simplify how we set the target triple

Instead of using TARGET_TRIPLE, which is always set to LLVM_DEFAULT_TARGET_TRIPLE,
use that variable directly to populate the various XXXX_TARGET_TRIPLE
variables in the runtimes.

This re-applies 77396bbc and 5099e015, which were reverted in
850b57c5 because they broke the build.

Differential Revision: https://reviews.llvm.org/D106009