Recognize -arch x86-64 too
Use k8 instead of x86_64 for gcc's -mtune

(gcc doesn't accept x86_64 and it says x86-64 is deprecated)
Fix missing entries in gcc_architectures() for FB_CPUTYPE_32/64

(they won't be used because FB_CPUTYPE_32/64 should cause the cputype to
be remapped to the default; but still the gcc_architectures() table should
still contain the entries, so "native" still is at the correct offset)
Fix -arch no longer being passed on to the compiler
Improve -arch native handling by using fb_CpuDetect()
Add the __FB_64BIT__ #define, defined only for 64bit targets