Adds conditional exports
- __fixkfti
- __fixunskfti
- __floattikf
- __negkf2
- __mulkc3
- __divkc3
- __powikf2
and adjusts tools/gen_stubs.zig.
From https://gcc.gnu.org/onlinedocs/gcc/Floating-Types.html:
"When long double transitions to __float128 on PowerPC in the future,
__ibm128 will remain for use in conversions between the two types."
Hence `__extendkftf2` and `__trunctfkf2` for conversion are superfluous
and only using f128 for `kf` routines is justified.
Closes#16057.
f043071cdf