Port to 3.1 - Fix JIT_CheckedWriteBarrier on macOS (#28051)

In a change to enable Mojave hardened runtime support that was made last
year, a bug was introduced into the JIT_CheckedWriteBarrier. A
conditional relative jump before an updated piece of code that was jumping
after that piece of code was not updated and ended up jumping into the
middle of an instruction. Since that condition occurs only with specific
memory layout and it is very rare, that problem was not discovered until
now.

src/vm/amd64/jithelpers_fast.S

@@ -33,8 +33,13 @@ LEAF_ENTRY JIT_CheckedWriteBarrier, _TEXT
         // See if this is in GCHeap
         PREPARE_EXTERNAL_VAR g_lowest_address, rax
         cmp     rdi, [rax]
+#ifdef FEATURE_WRITEBARRIER_COPY
+        // jb      NotInHeap
+        .byte 0x72, 0x12
+#else
         // jb      NotInHeap
         .byte 0x72, 0x0e
+#endif
         PREPARE_EXTERNAL_VAR g_highest_address, rax
         cmp     rdi, [rax]