ldflags=* are compiler flags, not linker flags (#43820)

We run `extend spack_flags_list SPACK_LDFLAGS` for `$mode in ld|ccld`.

That's problematic, cause `ccld` needs `-Wl,--flag` whereas `ld` needs
`--flag` directly. Only `-L` and `-l` are common to compiler & linker.

In all build systems `LDFLAGS` is for the compiler not the linker, cause
any linker flag `-x` can be passed as a compiler flag `-Wl,-x`, and there
are many compiler flags that affect the linker invocation, like `-fopenmp`,
`-fuse-ld=`, `-fsanitize=` etc.

So don't pass `LDFLAGS` to the linker directly.

This way users can set `ldflags: -Wl,--allow-shlib-undefined` in compilers.yaml
to work around an issue where the linker tries to resolve the `libcuda.so.1`
stub lib which cannot be located by design in `cuda`.

lib/spack/env/cc

@@ -755,7 +755,7 @@ esac
 
 # Linker flags
 case "$mode" in
-    ld|ccld)
+    ccld)
         extend spack_flags_list SPACK_LDFLAGS
         ;;
 esac

lib/spack/spack/test/cc.py

@@ -127,7 +127,7 @@
 spack_cflags = ["-Wall"]
 spack_cxxflags = ["-Werror"]
 spack_fflags = ["-w"]
-spack_ldflags = ["-L", "foo"]
+spack_ldflags = ["-Wl,--gc-sections", "-L", "foo"]
 spack_ldlibs = ["-lfoo"]
 
 lheaderpad = ["-Wl,-headerpad_max_install_names"]
@@ -279,7 +279,6 @@ def test_ld_flags(wrapper_environment, wrapper_flags):
         test_args,
         ["ld"]
         + test_include_paths
-        + [spack_ldflags[i] + spack_ldflags[i + 1] for i in range(0, len(spack_ldflags), 2)]
         + test_library_paths
         + ["--disable-new-dtags"]
         + test_rpaths
@@ -307,13 +306,14 @@ def test_cc_flags(wrapper_environment, wrapper_flags):
         [real_cc]
         + target_args
         + test_include_paths
-        + [spack_ldflags[i] + spack_ldflags[i + 1] for i in range(0, len(spack_ldflags), 2)]
+        + ["-Lfoo"]
         + test_library_paths
         + ["-Wl,--disable-new-dtags"]
         + test_wl_rpaths
         + test_args_without_paths
         + spack_cppflags
         + spack_cflags
+        + ["-Wl,--gc-sections"]
         + spack_ldlibs,
     )
 
@@ -325,12 +325,13 @@ def test_cxx_flags(wrapper_environment, wrapper_flags):
         [real_cc]
         + target_args
         + test_include_paths
-        + [spack_ldflags[i] + spack_ldflags[i + 1] for i in range(0, len(spack_ldflags), 2)]
+        + ["-Lfoo"]
         + test_library_paths
         + ["-Wl,--disable-new-dtags"]
         + test_wl_rpaths
         + test_args_without_paths
         + spack_cppflags
+        + ["-Wl,--gc-sections"]
         + spack_ldlibs,
     )
 
@@ -342,13 +343,14 @@ def test_fc_flags(wrapper_environment, wrapper_flags):
         [real_cc]
         + target_args
         + test_include_paths
-        + [spack_ldflags[i] + spack_ldflags[i + 1] for i in range(0, len(spack_ldflags), 2)]
+        + ["-Lfoo"]
         + test_library_paths
         + ["-Wl,--disable-new-dtags"]
         + test_wl_rpaths
         + test_args_without_paths
         + spack_fflags
         + spack_cppflags
+        + ["-Wl,--gc-sections"]
         + spack_ldlibs,
     )