u128_divide_sparc get build into rust stdlib for x86_64-pc-windows-msvc target

[klensy]

Checked exported symbols for std-[hash].dll for current stable and nightly and found _ZN17compiler_builtins3int19specialized_div_rem8delegate17u128_divide_sparc17hd4dfbbe4547e607fE inside, i guess #393 is source of this.

I guess sparc related functions shouldn't get build into other os stdlib.

[Amanieu]

cc @AaronKutch

[AaronKutch]

There are a number of hidden function specializations (another example is usize_leading_zeros_riscv) which are public so that they can get tested by the test suite which is in a separate crate. The CI does not have SPARC or RISCV which is why these are always enabled. I think I once wanted to make a feature flag for making these functions public, that would only be enabled by the testcrate. I can't remember why I never did it, I will open a PR and try doing it again.