This commit moves the write to FS and GS base, allowing for a much
more efficient write to GS base under x86-64 SMP. When writing
GS base was in Arch_switchToThread it was neccessary to write to
an MSR such that when swapgs was performed on kernel exit the new
value of GS base would be retrieved. Unfortunately writing to an
MSR is very expensive and we would much prefer to use the writegsbase
instructions instead. By moving this code to restore user context
we are able to call swapgs earlier and then use the normal
writegsbase instruction
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