[runtimes] Simplify how we set the target triple

[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 77396bbc98 and 5099e01568, which were reverted in
850b57c5fbe because they broke the build.

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