[arm64] Fix assembler native test failures.

We were attempting to assemble code into the MacroAssembler buffer after
executing it, without resetting the permissions. As a result, tests that
are using START/END multiple times were failing.

Change-Id: Id84c6a07212a869f98edbd33d86ff70ee6c819db
Reviewed-on: https://chromium-review.googlesource.com/939388Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Cr-Commit-Position: refs/heads/master@{#51901}

test/cctest/cctest.h

@@ -568,6 +568,13 @@ static inline void MakeAssemblerBufferExecutable(uint8_t* buffer,
   CHECK(result);
 }
 
+static inline void MakeAssemblerBufferWritable(uint8_t* buffer,
+                                               size_t allocated) {
+  bool result = v8::internal::SetPermissions(buffer, allocated,
+                                             v8::PageAllocator::kReadWrite);
+  CHECK(result);
+}
+
 static v8::debug::DebugDelegate dummy_delegate;
 
 static inline void EnableDebugger(v8::Isolate* isolate) {

test/cctest/test-assembler-arm64.cc

@@ -182,13 +182,13 @@ static void InitializeVM() {
                       v8::internal::CodeObjectRequired::kYes);   \
   RegisterDump core;
 
-#define RESET()                                                                \
-  __ Reset();                                                                  \
-  /* Reset the machine state (like simulator.ResetState()). */                 \
-  __ Msr(NZCV, xzr);                                                           \
+#define RESET()                                                \
+  MakeAssemblerBufferWritable(buf, allocated);                 \
+  __ Reset();                                                  \
+  /* Reset the machine state (like simulator.ResetState()). */ \
+  __ Msr(NZCV, xzr);                                           \
   __ Msr(FPCR, xzr);
 
-
 #define START_AFTER_RESET()                                                    \
   __ PushCalleeSavedRegisters();