[build] Check RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES for libc also (#82561)

When checking whether we need to build libc-hdrgen, we need to check
LLVM_ENABLE_RUNTIMES and RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES, just
the former is not sufficient since libc may be enabled only for certain
targets.