-- cmake on mingw: use big-obj with gcc and clang

[VDanielEdwards]

Subject

Use -Wa,-mbig-obj to allow bigger object files generated by the assembler. This solved an issue that occured during testing of the MinGW builds.

Instructions for review / testing

Developer checklist (address before review)

• [ ] Changelog.md updated
• [ ] Prepared update for depending repositories
• [ ] Documentation updated (public API changes only)
• [ ] API docstrings updated (public API changes only)
• [ ] Rebase → commit history clean
• [x] Squash and merge → proper PR title

[VDanielEdwards]

Test results (for comparison, s is without big-obj, b is with big-obj):

-- dbg s gcc
real    0m3.616s
real    3m9.309s
4.6G    _b/dbg-s-mingw64-gcc

-- dbg s clang
real    0m6.910s
real    2m11.593s
2.7G    _b/dbg-s-mingw64-clang

-- dbg b gcc
real    0m4.017s
real    3m13.348s
4.6G    _b/dbg-b-mingw64-gcc

-- dbg b clang
real    0m3.819s
real    2m15.095s
2.7G    _b/dbg-b-mingw64-clang

-- rel s gcc
real    0m3.926s
real    2m10.050s
362M    _b/rel-s-mingw64-gcc

-- rel s clang
real    0m4.195s
real    1m46.945s
358M    _b/rel-s-mingw64-clang

-- rel b gcc
real    0m4.686s
real    2m8.707s
365M    _b/rel-b-mingw64-gcc

-- rel b clang
real    0m7.932s
real    1m46.193s
358M    _b/rel-b-mingw64-clang