i#4461: Skip stolen register on drcachesim filter barriers

Do not try to get the application value of the stolen register on the
jump barrier used for drreg parity in filtered drcachesim
instrumentation.

Enable thread filtering on non-x86: there is no missing support there.

Tested by locally enabling the tool.drcacheoff.burst_threadfilter test
(it is disabled for AArch64 because of the #2007 link failure in some
toolchains), working around #4468 with "-steal_register 25", and
confirming that the drreg failure disappears with the fix here.

This is needed for the forthcoming new global filter for drcachesim
(#4462), in addition to the thread filter feature.  An
enabled-on-AArch64 test should be added as part of that feature to
serve as a regression test here, if #2007 has not been resolved by
then to enable the thread filter test.

Issue: #4461, #4462
Fixes #4461

clients/drcachesim/tracer/tracer.cpp

@@ -298,10 +298,6 @@ drmemtrace_status_t
 drmemtrace_filter_threads(bool (*should_trace_thread)(thread_id_t tid, void *user_data),
                           void *user_value)
 {
-#ifndef X86
-    /* XXX i#2820: only x86 supports thread filtering for now. */
-    return DRMEMTRACE_ERROR_NOT_IMPLEMENTED;
-#endif
     if (should_trace_thread == NULL)
         return DRMEMTRACE_ERROR_INVALID_PARAMETER;
     /* We document that this should be called once at init time: i.e., we do not
@@ -700,7 +696,11 @@ insert_conditional_skip_target(void *drcontext, instrlist_t *ilist, instr_t *whe
 {
     if (reg_barrier != DR_REG_NULL) {
         for (reg_id_t reg = DR_REG_START_GPR; reg <= DR_REG_STOP_GPR; ++reg) {
-            if (reg != reg_barrier) {
+            /* We're not allowed to restore the stolen register this way, but drreg
+             * won't hand it out as a scratch register in any case, so we don't need
+             * a barrier for it.
+             */
+            if (reg != reg_barrier && reg != dr_get_stolen_reg()) {
                 drreg_status_t res =
                     drreg_get_app_value(drcontext, ilist, where, reg, reg);
                 if (res != DRREG_ERROR_NO_APP_VALUE && res != DRREG_SUCCESS)