Bug 451743 - On x86 processors, only use SSE2 if the processor supports it, otherwise default back to x87 FPU (r=gal, sr=mrbkap)

Bug 451580, fix string split assertions and return values. Tests fail with JIT on for other reasons, it seems. r=shaver

Add split built-in, also sort knownNatives for my own sanity (should sort builtins.tbl and jsbuiltins.cpp by function while at it, but too tired).

If we expect a concrete value but see an undefined come into the trace, convert undefined to the concerete value.