Revert "[CMake] Use LLVM_TARGET_TRIPLE in runtimes"

This reverts commit bec8a372.

This causes many of these errors to appear when rebuilding runtimes part
of fuchsia's toolchain:

ld.lld: error:
/usr/local/google/home/paulkirth/llvm-upstream/build/lib/x86_64-unknown-linux-gnu/libunwind.a(libunwind.cpp.o)
is incompatible with elf64-x86-64

This can be reproduced by making a complete toolchain, saving any source
file with no changes, then rerunning ninja distribution.