Compiler.default_libc

Some logic to detect what libc the c / cxx compilers use by default,
based on `-dynamic-linker`.

The function `compiler.default_libc()` returns a `Spec` of the form
`glibc@x.y` or `musl@x.y` with the `external_path` property set.

The idea is this can be injected as a dependency.

If we can't run the dynamic linker directly, fall back to `ldd` relative
to the prefix computed from `ld.so.`

etc/spack/defaults/packages.yaml

@@ -35,6 +35,7 @@ packages:
       java: [openjdk, jdk, ibm-java]
       jpeg: [libjpeg-turbo, libjpeg]
       lapack: [openblas, amdlibflame]
+      libc: [glibc, musl]
       libgfortran: [ gcc-runtime ]
       libglx: [mesa+glx, mesa18+glx]
       libifcore: [ intel-oneapi-runtime ]