sparc: Return debug faults from unimplemented instructions.

sparc: Return debug faults from unimplemented instructions.

These had been marked as non-speculative so that their execute
functions would only be called if the instruction should really be
executed. Instead, we can return faults which will cause the same
behavior when the instruction is committed and let the instruction
execute as normal.

Change-Id: I39fa5073e93399424144724b99bdc12070e42286
Reviewed-on: https://gem5-review.googlesource.com/5465
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>