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`.
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