[CMake] Setting the LLVM_TARGET_TRIPLE macro based on the LLVM_DEFAULT_TARGET_TRIPLE

After D137870, LLVM_TARGET_TRIPLE is no longer defined on the runtime
path into compiler-rt. This patch creates a common block of code to set
LLVM_TARGET_TRIPLE equal to the default for both the llvm- and runtime-
paths.

Differential Revision: https://reviews.llvm.org/D138864