Add support for RD/WR FS/GSBASE

[Andi Kleen]

This adds kernel support for some Intel instructions that
allow fast access to the FS and GS 64bit base. They need
some changes to entry_64.S because they allow user
to fully control the GS base.

Advantages:
- NMIs (and other "paranoid" interrupts) avoid
one RDMSR which makes them faster
- User space can use these instructions, mainly
for efficient context switching with user thread libraries
- Context switches do not need to use MSR writes
anymore to context switch FS/GS base >4GB. This 
will speed up applications that have enough thread
local data that it won't fit below 4GB.
- User space can use GS efficiently as an additional
global pointer register

I also included one minor (unrelated) optimization to 
disable an unneeded old SWAPGS workaround.

-Andi

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