Fix #5459

This combines the .o emission code from #5787 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.
Add Please delete or regenerate sys.{so,dll,dylib}'' to all the error messages
Move -D_FILE_OFFSET_BITS back to CFLAGS

These should really go in CPPFLAGS, but apparently on mingw, there's defines
in the stdio headers that rename the function (rather than doing it on a symbol lelve), so we can't use this when we include LLVM headers. Anyway, as long as
we don't call the IO functions from C++ we should be fine.
Add newlines to error messages
cpuid -> jl_cpuid
Move jl_cpuid
Merge pull request #6220 from JuliaLang/kf/cpuid

Fix #5459