[runtimes] Fix incorrect comment about the purpose of LLVM_DEFAULT_TARGET_TRIPLE

5beec6fb added LLVM_DEFAULT_TARGET_TRIPLE to the runtimes build with
a comment, however I believe that comment had been copied from the LLVM
build tree. In the context of the runtimes, LLVM_DEFAULT_TARGET_TRIPLE
is used to set what targets we are building for, not the target for which
we "generate code".

Differential Revision: https://reviews.llvm.org/D114007