[openmp][arm64ec] Fix arm64x when PER_TARGET_RUNTIME_DIR=On (#207747)

This patch fixes an issue where the combination of LIBOMP_ENABLE_ARM64X
and LLVM_PER_TARGET_RUNTIME_DIR=On led to only the arm64 library being
installed. With this fix, the arm64ec and arm64 .lib files are both
installed, and the arm64x dll is the one installed to the binary dir.

GitOrigin-RevId: 0d9f3f41d6a45e0673c04a01e64d24a76ccffe60