Preserve required MS x64 call stack alignment when flushing registers.

Source/platform/heap/asm/SaveRegisters_x86.asm

 ;; Copyright (C) 2013 Google Inc. All rights reserved.
 ;;
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 ;; modification, are permitted provided that the following conditions are
 ;; met:
 ;;
 ;;     * Redistributions of source code must retain the above copyright
 ;; notice, this list of conditions and the following disclaimer.
 ;;     * Redistributions in binary form must reproduce the above
 ;; copyright notice, this list of conditions and the following disclaimer
@@ skipping 83 matching lines @@
         ;; Pop the callee-saved registers. None of them were
         ;; modified so no restoring is needed.
         add rsp, 56
         ret
 
 %elif X64WIN
 
 mangle(pushAllRegisters):
         ;; Push all callee-saves registers to get them
         ;; on the stack for conservative stack scanning.
+        ;; There is an 8-byte return address on the stack and we push
+        ;; 72 bytes which maintains the required 16-byte stack alignment
+        ;; at the call.
+        push 0
         push rsi
         push rdi
         push rbx
         push rbp
         push r12
         push r13
         push r14
         push r15
         ;; Pass the two first arguments unchanged (rcx, rdx)
         ;; and the stack pointer after pushing callee-saved
         ;; registers to the callback.
         mov r9, r8
         mov r8, rsp
         call r9
         ;; Pop the callee-saved registers. None of them were
         ;; modified so no restoring is needed.
-        add rsp, 64
+        add rsp, 72
         ret
 
 %elif IA32
 
 mangle(pushAllRegisters):
         ;; Push all callee-saves registers to get them
         ;; on the stack for conservative stack scanning.
         ;; We maintain 16-byte alignment at calls (required on
         ;; Mac). There is a 4-byte return address on the stack
         ;; and we push 28 bytes which maintains 16-byte alignment
@@ skipping 15 matching lines @@
         ;; so we do not need to restore them.
         add esp, 28
         ret
 
 
 %elif ARM
 %error "Yasm does not support arm. Use SaveRegisters_arm.S on arm."
 %else
 %error "Unsupported platform."
 %endif