x86: Add COND_VZEROUPPER that can replace vzeroupper if no `ret`

x86: Add COND_VZEROUPPER that can replace vzeroupper if no `ret`

The RTM vzeroupper mitigation has no way of replacing inline
vzeroupper not before a return.

This can be useful when hoisting a vzeroupper to save code size
for example:

```
L(foo):
	cmpl	%eax, %edx
	jz	L(bar)
	tzcntl	%eax, %eax
	addq	%rdi, %rax
	VZEROUPPER_RETURN

L(bar):
	xorl	%eax, %eax
	VZEROUPPER_RETURN
```

Can become:

```
L(foo):
	COND_VZEROUPPER
	cmpl	%eax, %edx
	jz	L(bar)
	tzcntl	%eax, %eax
	addq	%rdi, %rax
	ret

L(bar):
	xorl	%eax, %eax
	ret
```

This code does not change any existing functionality.

There is no difference in the objdump of libc.so before and after this
patch.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>