x86: cpufunc: Add rdtsc_ordered()

[adam_fenn.io]

Summary

Add a variant of rdtsc() that performs the ordered version of rdtsc
appropriate for the invoking x86 variant.

Test Plan

Tested as part of testing D29733.

[kib]

sys/x86/x86/rdtsc_ordered.c

54

I would add rdtsc_ordered_*fence() functions to cpufunc.h as well.

63

I suppose this branch did not caused a warning from the compiler? I suggest to add {} around the body of the case. Or write it as

return (cpu_is_amd ? rdtsc_ordered_mfence : rdtsc_ordered_lfence);

[adam_fenn.io]

Address @kib's feedback.

[kib]

sys/conf/files.x86

322

Drop this file at all.

sys/i386/include/cpufunc.h

836

Move this to x86/include/md_var.h

sys/x86/x86/rdtsc_ordered.c

55

Move this to x86/cpu_machdep.c

[adam_fenn.io]

Address @kib's feedback.