x64: fix off-by-one in stack alignment

Harmless.

arch/x64/entry.S

@@ -106,7 +106,7 @@ stack_trampoline:  # %rsi=arg, %rdi=func, %rdx=stack
 	.cfi_startproc simple
 	.cfi_def_cfa %rsp, 0
 	# align the stack at 16 bytes for sse
-	andq $-15, %rdx
+	andq $-16, %rdx
 	sub $8, %rdx
 	mov %rsp, -8(%rdx)
 	lea -8(%rdx), %rsp