[PATCH, i386]: Fix PR target/35553, -fkeep-inline-functions and -O errors our in SSE headers

[Paolo Bonzini]

> The one complexity with the "extern inline" solution (as mentioned in the
> comments on that PR) is that you really do need extern definitions of the
> functions (presumably in libgcc), for -fno-inline or in case someone
> references an _mm_* function as a pointer-to-function.

I think __artificial__ should forbid taking pointer-to-functions (not 
that it does!).

Paolo