fastmodel: Update the IRIS ThreadContext base class.
fastmodel: Update the IRIS ThreadContext base class.

The syscall() method has been removed, and HTM related methods have
been added.

Change-Id: I796c1a554bfd4b1ee01a62c9c7ad403dd699cc0f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35038
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Maintainer: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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