Tighten arm32/arm64 write barrier kill reg sets

The JIT write barrier helpers have a custom calling convention that
avoids killing most registers. The JIT was not taking advantage of
this, and thus was killing unnecessary registers when a write barrier
was necessary. In particular, some integer callee-trash registers
are unaffected by the write barriers, and no floating-point register
is affected.

Also, I got rid of the `FEATURE_WRITE_BARRIER` define, which is always
set. I also put some code under `LEGACY_BACKEND` for easier cleanup
later. I removed some unused defines in target.h for some platforms.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/jit/emitarm64.cpp b/src/jit/emitarm64.cpp
index fe690d2abb..dfca2cdd00 100644
--- a/src/jit/emitarm64.cpp
+++ b/src/jit/emitarm64.cpp
@@ -7356,6 +7356,16 @@ void emitter::emitIns_Call(EmitCallType          callType,
         {
             savedSet |= RBM_PROFILER_RET_SCRATCH;
         }
+
+#ifdef DEBUG
+        if (emitComp->verbose)
+        {
+            printf("NOGC Call: savedSet=");
+            printRegMaskInt(savedSet);
+            emitDispRegSet(savedSet);
+            printf("\n");
+        }
+#endif
     }
     else
     {