Fix cfloat return on x86_64: only perform ABI transformation for non-extern(D) functions. There's no need to waste cycles with extern(D), which we get to define ourselves. Fixes tests/mini/asm8.d. (Since the asm abiret code already assumed {xmm0, xmm1} returns)

#ifndef _GNUC_H
#define _GNUC_H 1

int memicmp(const char *s1, const char *s2, int n);
int stricmp(const char *s1, const char *s2);

#endif