i#5036 A64 scatter/gather, part 9: State restore (#6753)

Adds support for restoring register app state when a fault is triggered
by one of the instructions in a scatter/gather expansion sequence
emitted by drx_expand_scatter_gather().

Also extends the existing scatter/gather expansion tests to check the
value of the FFR register, and run the test instructions with different
elements rigged to fault so we can test the state restoration behaviour.

Support for the first-faulting and non-faulting variants (ldff1*,
ldnf1*) will be added in future commits.

Issue: #5036

clients/drcachesim/tools/invariant_checker.cpp

@@ -846,7 +846,14 @@ invariant_checker_t::parallel_shard_memref(void *shard_data, const memref_t &mem
                 // DR hands us a different address for sysenter than the
                 // resumption point.
                 shard->last_signal_context_.pre_signal_instr.memref.instr.type ==
-                    TRACE_TYPE_INSTR_SYSENTER;
+                    TRACE_TYPE_INSTR_SYSENTER // clang-format off
+                // The AArch64 scatter/gather expansion test skips faulting
+                // instructions in the signal handler by incrementing the PC.
+                IF_AARCH64(||
+                           (knob_test_name_ == "scattergather" &&
+                            memref.instr.addr ==
+                                shard->last_signal_context_.xfer_int_pc + 4));
+            // clang-format on
             report_if_false(
                 shard,
                 kernel_event_marker_equality ||