* spack.compiler: cache output
* compute libc from the dynamic linker at most once per spack process
* wrap compiler cache entry in class, add type hints
* test compiler caching
* ensure tests do not populate user cache, and fix 2 tests
* avoid recursion: cache lookup -> compute key -> cflags -> real_version -> cache lookup
* allow compiler execution in test that depends on get_real_version
c6997e11a7
