radv: Use library mtime for cache UUID.

radv: Use library mtime for cache UUID.

We want to also invalidate the cache when LLVM gets changed. As the
specific LLVM revision is not fixed at build time, we will need to
check at runtime. Computing a checksum for LLVM is going to be very
expensive, so just use the mtime.

Tested on my computer that the returned DSO for the LLVM symbol is
actually the LLVM DSO.

Signed-off-by: Bas Nieuwenhuizen <basni@google.com>