x86: Add definitions for Intel PT XSAVE state

[bnovkov]

Summary

This change adds definitions for the Intel Processor Trace XSAVE state, according to the specification in the Intel SDM, Vol. 1 13-13.

[kib]

Since XSAVES always use the compact format, and since we already handle the save area format differently for XSAVE. I suggest to not add the pt_save_area structure definition as is. If we ever want to add yet another system feature from XSAVES, using such structures cause exponential explosion of the needed definitions. It is better to define the pt_ext_area, and then calculate the position of it in the save area based on the feature bitmap.

sys/x86/include/fpu.h

199

This probably should be struct fpusave, but see below.