gcc-runtime: add separate package for gcc runtime libs

The gcc-runtime package adds a separate node for gcc's dynamic runtime
libraries.

This should help with:

1. binary caches where rpaths for compiler support libs cannot be
   relocated because the compiler is missing on the target system
2. creating "minimal" container images

The package is versioned like `gcc` (in principle it could be
unversioned, but Spack doesn't always guarantee not mixing compilers)

share/spack/gitlab/cloud_pipelines/stacks/e4s-cray-rhel/spack.yaml

@@ -33,6 +33,8 @@ spack:
     elfutils:
       variants: +bzip2 ~nls +xz
       require: "%gcc"
+    gcc-runtime:
+      require: "%gcc"
     hdf5:
       variants: +fortran +hl +shared
     libfabric:

share/spack/gitlab/cloud_pipelines/stacks/e4s-oneapi/spack.yaml

@@ -17,6 +17,8 @@ spack:
       variants: +mpi
     elfutils:
       variants: +bzip2 ~nls +xz
+    gcc-runtime:
+      require: "%gcc"
     hdf5:
       require: "%gcc"
       variants: +fortran +hl +shared