Use VERW to mitigate the RFDS (Register File Data Sampling) vulnerability

Use VERW to mitigate the RFDS (Register File Data Sampling) vulnerability
present in Intel Atom CPUs, reordering some ASM in return-to-userspace and
start/resume-vmx-guest to reduce the number of kernel values still live in
registers when VERW is used.  This mitigation requires updated firmware which
has affected CPUs report RFDS_CLEAR in dmesg.

Firmware packaging by jsg@ and sthen@
Logic for interpreting intel's flags by jsg@ after lots of discussion
   between him, deraadt@, and I
ok deraadt@