[Python-ideas] Add processor generation to wheel metadata

[Ivan Pozdeev]

Generally, packages are compiled for the same processor generation as 
the corresponding Python.
But not always -- e.g. NumPy opted for SSE2 even for Py2 to work around 
some compiler bug
(https://github.com/numpy/numpy/issues/6428).
I was bitten by that at an old machine once and found out that there is 
no way for `pip' to have checked for that.
Besides, performance-oriented packages like the one mentioned could 
probably benefit from newer instructions.

Regarding identifiers:
gcc, cl and clang all have their private directories of generation 
identifiers:
https://gcc.gnu.org/onlinedocs/gcc-4.7.1/gcc/i386-and-x86_002d64-Options.html
https://msdn.microsoft.com/en-us/library/7t5yh4fd.aspx
https://clang.llvm.org/doxygen/Driver_2ToolChains_2Arch_2X86_8cpp_source.html

Linux packages typically use gcc's ones. Clang generally follows in 
gcc's footsteps and accepts cl's IDs, too, as aliases.

So, using the IDs of whatever compiler is used to build the package 
(i.e. most likely the canonical compiler for CPython for that platform) 
looks like the simple&stupid(r) way - we can just take the value of the 
"march" argument.


The tricky part is mapping the system's processor to an ID when checking 
compatibility: the logic will have to keep a directory (that's the job 
of `wheel' package maintainers though, I guess).
I also guess that there are cases where there's no such thing as _the_ 
system's processor.

-- 
Regards,
Ivan