Allow PROT_EXEC on the zygote mappings when falling back to ashmem

Because we now protect the mappings before creating the executable one,
we need to also allow PROT_EXEC on an ashmem region.

Test: manually change code to use ashmem, build, see that JIT is
created.
Bug: 202999239

Change-Id: I9f2944c45f4ff607c7e341552c82138bfe13cca4
diff --git a/runtime/jit/jit_memory_region.cc b/runtime/jit/jit_memory_region.cc
index 58a4041..1592767 100644
--- a/runtime/jit/jit_memory_region.cc
+++ b/runtime/jit/jit_memory_region.cc
@@ -552,7 +552,7 @@
       return false;
     }
   } else {
-    palette_status_t status = PaletteAshmemSetProtRegion(fd, PROT_READ);
+    palette_status_t status = PaletteAshmemSetProtRegion(fd, PROT_READ | PROT_EXEC);
     if (status != PALETTE_STATUS_OK) {
       CHECK_EQ(status, PALETTE_STATUS_CHECK_ERRNO);
       std::ostringstream oss;