_toolchain: Properly set compiler/linker executables on Gentoo

[ajelinski]

The test_toolchain_cxx.py tests definitely use compiler and linker set
with _so_cxx-suffixed parameters. After this change tests use a proper
executable instead of c++.

[whitequark]

The tests are failing.

[ajelinski]

At first I was extremely confused by the error:

ValueError: unknown executable 'compiler_so_cxx' for class UnixCCompiler

as it worked and helped in my case.

After doing some research it seems the whole thing is quite Gentoo-specific. Python is built there after applying quite a few patches and adding {compiler,linker}_so_cxx to UnixCCompiler is in one of these patches: https://github.com/OpenMandrivaAssociation/python/blob/ccf96eae53697ec09e8d03ad46cd8be4ce044548/0005-Improve-distutils-C-support.patch#L243-L246 (that's not the latest version of the patch but these lines haven't changed).

I've modified the commit to try to set them but skip raising the exception if not available.

[codecov]

Codecov Report

Merging #632 (4f53bf6) into master (23a44f3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #632      +/-   ##
==========================================
+ Coverage   81.46%   81.47%   +0.01%     
==========================================
  Files          49       49              
  Lines        6446     6450       +4     
  Branches     1287     1287              
==========================================
+ Hits         5251     5255       +4     
  Misses       1008     1008              
  Partials      187      187              

Impacted Files	Coverage Δ
nmigen/_toolchain/cxx.py	100.00% <100.00%> (ø)

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[whitequark]

LGTM, thank you!