compiler wrapper: parse Wl and Xlinker properly (#35912)

Two fixes:

1. `-Wl,a,b,c,d` is a comma separated list of linker arguments, we
   incorrectly assume key/value pairs, which runs into issues with for
   example `-Wl,--enable-new-dtags,-rpath,/x`
2. `-Xlinker,xxx` is not a think, so it shouldn't be parsed.

lib/spack/spack/test/build_environment.py

@@ -127,13 +127,13 @@ def test_static_to_shared_library(build_environment):
         "linux": (
             "/bin/mycc -shared"
             " -Wl,--disable-new-dtags"
-            " -Wl,-soname,{2} -Wl,--whole-archive {0}"
+            " -Wl,-soname -Wl,{2} -Wl,--whole-archive {0}"
             " -Wl,--no-whole-archive -o {1}"
         ),
         "darwin": (
             "/bin/mycc -dynamiclib"
             " -Wl,--disable-new-dtags"
-            " -install_name {1} -Wl,-force_load,{0} -o {1}"
+            " -install_name {1} -Wl,-force_load -Wl,{0} -o {1}"
         ),
     }
 

lib/spack/spack/test/cc.py

@@ -342,6 +342,16 @@ def test_fc_flags(wrapper_environment, wrapper_flags):
     )
 
 
+def test_Wl_parsing(wrapper_environment):
+    check_args(
+        cc,
+        ["-Wl,-rpath,/a,--enable-new-dtags,-rpath=/b,--rpath", "-Wl,/c"],
+        [real_cc]
+        + target_args
+        + ["-Wl,--disable-new-dtags", "-Wl,-rpath,/a", "-Wl,-rpath,/b", "-Wl,-rpath,/c"],
+    )
+
+
 def test_dep_rpath(wrapper_environment):
     """Ensure RPATHs for root package are added."""
     check_args(cc, test_args, [real_cc] + target_args + common_compile_args)