LLVM backend: Fix fabs() builtin declaration mixup
LLVM backend: Implement float-to-int rounding using llvm.nearbyint()
C backend: Fix missing ';' (5db8f8aa regression)
C backend: Only use asm() hack for STDCALL_MS on Win32 (see also 5db8f8aa)
C backend: Use __builtin_nearbyint() instead of __builtin_rint()

I'm hoping it's the same but without floating point exception stuff, so
hopefully closer to the ASM backend.