Fix #5459

This combines the .o emission code from #5787 (without the .o loading code that required LLVM3.5) with some error checking code for cpuid mismatch. When releasing a binary, set JULIA_CPU_TARGET to "core2" (we discussed that this is a reasonable minimum) in your Make.user and everything should work just fine.

@JeffBezanson 
@vtjnash (if you want to look it over)