VM: fix jit-signal-handler-prolog/epilog to account for the home space

VM: fix jit-signal-handler-prolog/epilog to account for the home space

Windows 64bit abi requires callers to reserve 32 bytes of home space in
the stack frame which the callee is free to clobber. Previous versions
of VS didn't compile code to take advantage of the home space so it
worked fined. VS2015 however, is using the home space which causes
registers and flags that were supposed to be saved to be
overwritten. The fix is to put a little extra empty space at the bottom
of the stack frame.