compiler_rt: add PPC64 __fixkfti and __floattikf alias

[matu3ba]

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[matu3ba]

~~Looks like "libgcc code is the abi", since I can not find __fixkfti and __floattikf in Power ISA TM Version 3.1B. Note, that this assumes long double is f128, which is in general not the case.~~

UPDATE: See also #16054.

TODO:

• [x] ./deb/bin/zig build test-std -fqemu -Dtarget=powerpc-linux with target PR works
• [x] tools/gen_stubs.zig:582 Adjust blacklisted_symbols.
• [x] update upstream docs: https://github.com/matu3ba/matu3ba.github.io/blob/master/crt/crt_unofficial_zig_docs.md, https://github.com/matu3ba/matu3ba.github.io/blob/master/crt/crt_float_routines.md

[andrewrk]

Great work!

[matu3ba]

Thanks!