clang i686-w64-windows-gnu weak reference mangling

[jeremyd2019]

Sorry, this is probably pretty esoteric...

I am attempting to build for i686-w64-windows-gnu, and it is failing to link. After some investigation I found the cause. Clang is hitting this case:

openlibm/src/cdefs-compat.h

Lines 61 to 66 in f052f42

	#elif defined(__clang__) /* CLANG */
	#ifdef __STDC__
	#define openlibm_weak_reference(sym,alias) \
	__asm__(".weak_reference " #alias); \
	__asm__(".set " #alias ", " #sym)
	#else

But, on x86 32-bit Windows, symbols for C functions are prefixed by _. Changing that so that sym and alias are prefixed by _ solves the linking there. That doesn't seems like it would work everywhere though.

In contrast, I think GCC would hit the case:

openlibm/src/cdefs-compat.h

Lines 73 to 75 in f052f42

	#define openlibm_weak_reference(sym,alias) \
	__asm__(".stabs \"_" #alias "\",11,0,0,0"); \
	__asm__(".stabs \"_" #sym "\",1,0,0,0")

Which always includes _ prefixes,,,

See also https://github.com/mingw-w64/mingw-w64/blob/6b62cf66832bd158c67662cd5566413baf35f20e/mingw-w64-headers/crt/_mingw_mac.h#L130 and the lines immediately before that figure out whether the _ is required or not

[ViralBShah]

Could you provide a PR here?