spack.compiler/spack.util.libc: add caching (#47213)

* 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

lib/spack/docs/conf.py

@@ -221,6 +221,7 @@ def setup(sphinx):
     ("py:class", "spack.filesystem_view.SimpleFilesystemView"),
     ("py:class", "spack.traverse.EdgeAndDepth"),
     ("py:class", "archspec.cpu.microarchitecture.Microarchitecture"),
+    ("py:class", "spack.compiler.CompilerCache"),
     # TypeVar that is not handled correctly
     ("py:class", "llnl.util.lang.T"),
 ]