Reorganize libclang_rt Makefile and make more lib/arch combos available

Upstream has made more clang runtime libraries available for more
architectures, so add them. To make this easier, split up subdir lists
into functional parts (asan, tsan, etc), and put each architecture into
its own .if block.

Effectively, this adds the following libraries for aarch64: asan, cfi,
fuzzer, msan, safestack, stats, tsan, ubsan, xray.

PR: 262706
MFC after: 3 days