[CMake][runtimes] Check LLVM_ENABLE_PROJECTS for libc (#76845)

Only some targets may be building llvm-libc in which case the top-level
LLVM_ENABLE_RUNTIMES variable won't contain libc. Rather, we can check
LLVM_ENABLE_PROJECTS since libc is required to be included there for
libc-hdrgen to be built (and when LLVM_ENABLE_RUNTIMES contains libc, we
automatically include libc in LLVM_ENABLE_PROJECTS as well).