x86, xsave: context switch support using xsave/xrstor

Uses xsave/xrstor (instead of traditional fxsave/fxrstor) in context switch
when available.

Introduces TS_XSAVE flag, which determine the need to use xsave/xrstor
instructions during context switch instead of the legacy fxsave/fxrstor
instructions. Thread-synchronous status word is already in L1 cache during
this code patch and thus minimizes the performance penality compared to
(cpu_has_xsave) checks.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>