arch-arm: gdb support Thumb ISA

BaseRemoteGDB only checks if the breakpoint size equals to MachInst size.
However, there are two kinds of instruction size in ARM. We should allow
the 16-bits breakpoint for Thumb ISA.

Change-Id: I79c0e1503092ecf233c719f2e354739edb8e6b25
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52624
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
2 files changed