targets: first pass at target detection for linux

Add llnl.util.cpu_name, with initial support for detecting different
microarchitectures on Linux.  This also adds preliminary changes for
compiler support and variants to control the optimizatoin levels by
target.

This does not yet include translations of targets to particular
compilers; that is left to another PR.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

lib/spack/env/cc

@@ -32,6 +32,7 @@ parameters=(
     SPACK_CXX_RPATH_ARG
     SPACK_F77_RPATH_ARG
     SPACK_FC_RPATH_ARG
+    SPACK_TARGET_ARGS
     SPACK_SHORT_SPEC
     SPACK_SYSTEM_DIRS
 )
@@ -78,7 +79,7 @@ function system_dir {
 }
 
 for param in "${parameters[@]}"; do
-    if [[ -z ${!param} ]]; then
+    if [[ -z ${!param+x} ]]; then
         die "Spack compiler must be run from Spack! Input '$param' is missing."
     fi
 done
@@ -373,6 +374,7 @@ case "$mode" in
             CXX)
                 flags=("${flags[@]}" "${SPACK_CXXFLAGS[@]}") ;;
         esac
+        args=(${SPACK_TARGET_ARGS[@]} "${args[@]}")
         ;;
 esac