x86/fpu: extract extended related code into xstate.h and xstate.c

x86/fpu: extract extended related code into xstate.h and xstate.c

Current extended code is mixing with FPU code in i387.c. As part of
FPU code cleanup, this patch moves all extended state code into
independent files. Not much semantic are changed and most function
names are kept untouched, except for xsave() and xsaveopt(). These two
functions are combined into a single function.

Signed-off-by: Wei Huang <wei.huang2@amd.com>