You still need to know whether the C library expects stdcall, pascal, cdecl, several variants of fastcall, as well as a dozen less popular conventions - and that's just on x86.

Stdcall convention in C also often does some name mangling (leading underscore, trailing @ and the number of bytes on stack) but not always.

Also a C++ ABI would need to extend beyond simple calling conventions and also specify fixed behaviour of vtables, which i think compiler authors might balk at.