add toolchain option to customize linker command that compiler should use + use correct compiler options for using mold with GCC (WIP)

[boegel]

Usage to use mold as a linker:

toolchainopts = {'linker': 'mold'}

For non-GCC compilers (Clang, Intel) and GCC 12.1, this is equivalent with adding -fuse-ld=mold to $CFLAGS and friends.
For GCC < 12.1, this is equivalent to adding -B$EBROOTMOLD/libexec/mold to $CFLAGS and co.
See also classic way to use mold at https://github.com/rui314/mold#how-to-use.

Of course this also requires that mold is actually available, so it should be added as a build dependency (or be available system-wide).

Marked as WIP because:

• these change broke one of the tests for some reason (to be figured out);
• need to make sure that tests cover old/new GCC, Intel, Clang;
• using this with Intel compilers doesn't actually seem to work; tested with GSL 2.7:

  $ ls software/GSL/*/bin/gsl-histogram | xargs readelf -p .comment | egrep '^File|mold'
  File: software/GSL/2.7-ClangGCC-13.0.1-11.2.0/bin/gsl-histogram
    [    53]  mold 1.2.1 (compatible with GNU ld)
  File: software/GSL/2.7-GCC-11.3.0/bin/gsl-histogram
    [    2c]  mold 1.2.1 (compatible with GNU ld)
  File: software/GSL/2.7-intel-compilers-2022.1.0/bin/gsl-histogram
  

[boegel]

Based on https://www.intel.com/content/www/us/en/develop/documentation/oneapi-dpcpp-cpp-compiler-dev-guide-and-reference/top/compiler-reference/compiler-options/linking-or-linker-options/fuse-ld.html, the -fuse-ld compiler option only accepts bfd or gold, so -fuse-ld=mold doesn't work as expected.
Rather than producing an error when passing an unsupported value, it seems that the Intel compiler just silently ignores that option and continues with using its default linker instead. So we should probably resort to also using the -B workaround for the Intel compiler.

@rui314 Have you ever tested mold with the Intel compilers?

[rui314]

@boegel A few bugs have been filed to make mold work with ICC. We fixed them, and the bug reporters seemed happy, so I believe it works with ICC. But I haven't tried it myself.

[boegel]

@rui314 Is it correct that you need to use the -B option to make ICC use mold as linker (as opposed to -fuse-ld=mold)?
If so, I'll open a pull request to update mold's README file to mention that...

[rui314]

@boegel I don't actually know. If icc doesn't take -fuse-ld=mold, then the answer is probably yes though.

[boegel]

@boegel I don't actually know. If icc doesn't take -fuse-ld=mold, then the answer is probably yes though.

Well, it accepts the option, but then silently doesn't use mold at all (based on output of readelf -p .comment on the binary)