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