prefix symbols with _ for 32-bit x86 Windows

[jeremyd2019]

In a case that I believe can only be hit for Clang i686-*-windows-gnu (AKA MinGW), symbols in asm need to be prefixed with _. Fixes #237

[jeremyd2019]

I don't know about the non-__STDC__ case, or if it's even reachable for Clang i686-w64-windows-gnu

[ViralBShah]

@staticfloat Would you be able to take a look at this?

Also, it would be so much easier to merge these if we had CI working through gh actions.

[staticfloat]

I talked it over with @vtjnash and we both agree that supporting non-__STDC__ stuff is a waste of time; very unlikely that anyone is trying to build openlibm with such an archaic C compiler. Feel free to ignore that, and we will remove the branches in a future commit.

[ViralBShah]

This one didn't build on my mac - so I had to revert it.

src/s_isinf.c:66:1: error: expected ')'
openlibm_weak_reference(__isinff, isinff);
^
/Users/viral/repos/openlibm/src/cdefs-compat.h:69:32: note: expanded from macro 'openlibm_weak_reference'
    __asm__(".weak_reference " openlibm_symbol_prefix #alias); \
                               ^
src/s_isinf.c:66:1: note: to match this '('
/Users/viral/repos/openlibm/src/cdefs-compat.h:69:12: note: expanded from macro 'openlibm_weak_reference'
    __asm__(".weak_reference " openlibm_symbol_prefix #alias); \
           ^
src/s_isinf.c:66:1: error: expected ')'
openlibm_weak_reference(__isinff, isinff);
^
/Users/viral/repos/openlibm/src/cdefs-compat.h:70:21: note: expanded from macro 'openlibm_weak_reference'
    __asm__(".set " openlibm_symbol_prefix #alias ", " openlibm_symbol_prefix #sym)
                    ^
src/s_isinf.c:66:1: note: to match this '('
/Users/viral/repos/openlibm/src/cdefs-compat.h:70:12: note: expanded from macro 'openlibm_weak_reference'
    __asm__(".set " openlibm_symbol_prefix #alias ", " openlibm_symbol_prefix #sym)
           ^
2 errors generated.
make: *** [src/s_isinf.c.o] Error 1

[jeremyd2019]

Yeah, it appears that #undefing the symbol prefix does not work (I was worried about that, and my test only succeeded because of the typo).

[ViralBShah]

May need a new PR - and while I now have CI running, I don't think I am able to set up the windows CI correctly for x86.