[PAL/Linux-SGX] Use `XSAVEC` or `XSAVEOPT` instead of `XSAVE` instruction about gramine HOT 1 OPEN

Comments from an expert:

1. XSAVEC is only a drop-in replacement for XSAVE if XSAVEC and XRSTOR are the only instructions that read/write to that XSAVE area. There are some scenarios where software (e.g., an exception handler) may need to examine and possibly modify the XSAVE area. In these scenarios, XSAVEC (and also XSAVEOPT) cannot be used as a drop-in replacement for XSAVE because the format/layout of the XSAVE area will be different from XSAVE.

2. This statement is not quite accurate: “XSAVEOPT checks whether the XSAVE state was modified, by consulting the XSAVE area (restored previously with XRSTOR).” When XRSTOR is executed, it clears the processor’s “modified” tracker bits. As soon as some extended state is updated (e.g., an AVX register), the associated “modified” bit is set. Then when the next XSAVEOPT is executed to write to the same XSAVE area used by the most recent XRSTOR, XSAVEOPT consults with the tracker bits to determine what state has been modified, and thus what state must be updated in the XSAVE area. This behavior is described in Section 13.9 of the SDM, volume 1.

from gramine.