Save and restore ymm registers in signal handlers.

Save and restore ymm registers in signal handlers.

- Modified the CONTEXT structure for storing the upper 16 bytes of ymm registers
- Upon start of signal handler, ymmh data is copied from the native context to the CONTEXT structure, and a new flag is set to indicate that it has ymmh data
- Upon calling RtlRestoreContext, the new flag is checked, and ymmh data is restored into registers from the CONTEXT structure
- This change fixes only the Linux side for now.