Arm64: Ensure that csp is always aligned to 16 byte values even if jssp is not.

src/arm64/macro-assembler-arm64.h

Index: src/arm64/macro-assembler-arm64.h
diff --git a/src/arm64/macro-assembler-arm64.h b/src/arm64/macro-assembler-arm64.h
index 965d1dbfd9d5ef125b40d84ad83063079d40fcc3..177e4781aa604d2ba95b8353abd04f3b906e432c 100644
--- a/src/arm64/macro-assembler-arm64.h
+++ b/src/arm64/macro-assembler-arm64.h
@@ -778,12 +778,22 @@ class MacroAssembler : public Assembler {
   //
   // This is necessary when pushing or otherwise adding things to the stack, to
   // satisfy the AAPCS64 constraint that the memory below the system stack
-  // pointer is not accessed.
+  // pointer is not accessed.  The amount pushed will be increased as necessary
+  // to ensure csp remains aligned to 16 bytes.
   //
   // This method asserts that StackPointer() is not csp, since the call does
   // not make sense in that context.
   inline void BumpSystemStackPointer(const Operand& space);
 
+  // Re-synchronizes the system stack pointer (csp) with the current stack
+  // pointer (according to StackPointer()).  This function will ensure the
+  // new value of the system stack pointer is remains aligned to 16 bytes, and
+  // is lower than or equal to the value of the current stack pointer.
+  //
+  // This method asserts that StackPointer() is not csp, since the call does
+  // not make sense in that context.
+  inline void SyncSystemStackPointer();
+
   // Helpers ------------------------------------------------------------------
   // Root register.
   inline void InitializeRootRegister();