[WIP] allow selection of the compile/link toolchain for python_dist()

[cosmicexplorer]

WIP because there is absolutely no testing yet

Problem

There is currently no way for a user to affect which compiler or linker is run at any point in which our native toolchain is invoked. This adds that option for python_dist() targets.

Also, all the random stuff we add to the environment is just confusing distutils and making it produce compile errors in lots of internal usages of python_dist() -- until #6273 is merged, leaving the setup.py invocation environment mostly untouched allows these to succeed.

(explain the context of the problem and why you're making this change. include
references to all relevant github issues.)

Solution

(describe the modifications you've made.)

Result

(describe how your changes affect the end-user behavior of the system. this section is
optional, and should generally be summarized in the title of the pull request.)

[cosmicexplorer]

This is also obsolete as of #7126, and #6800 does the job of selecting the toolchain for the native backend now anyway.