Do "Return stack refilling", based on the "Return stack underflow" discussion

and its associated appendix at https://support.google.com/faqs/answer/7625886
This should address at least some cases of "SpectreRSB" and earlier
Spectre variants; more commits to follow.

The refilling is done in the enter-kernel-from-userspace and
return-to-userspace-from-kernel paths, making sure to do it before
unblocking interrupts so that a successive interrupt can't get the
CPU to C code without doing this refill.  Per the link above, it
also does it immediately after mwait, apparently in case the low-power
CPU states of idle-via-mwait flush the RSB.

ok mlarkin@ deraadt@

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