gh-95855: Refactor platform triplet detection code, add detection for MIPS soft float and musl libc (#107221)

- Move platform triplet detection code into Misc/platform_triplet.c
- Refactor MIPS detection, use defined(__mips64) to detect MIPS64
- Compute libc values in separate section
- Add detection for MIPS soft float
- Add detection for musl

musl supports SPE with its soft-float ABI:
https://git.musl-libc.org/cgit/musl/commit/?id=7be59733d71ada3a32a98622507399253f1d5e48

Original patch by Christian Heimes.

Co-authored-by: Christian Heimes <christian@python.org>
Co-authored-by: Erlend E. Aasland <erlend@python.org>