An instruction that references TLS, e.g.

MOVQ	0(TLS), AX

on some platforms (e.g. Android), or in shared mode, may be
translated to (assuming TLS offset already loaded to CX)

MOVQ	0(CX)(TLS*1), AX

which in turns translates to

movq	%fs:(%rcx), %rax

We have rejected non-zero offset for TLS reference, like 16(TLS).
Actually, the instruction can take offset, i.e. it is a valid
instruction for, e.g.,

movq	%fs:16(%rcx),%rcx

So, allow offset in TLS reference.

Change-Id: Iaf1996bad7fe874e0c298ea441af5acb136a4028
Reviewed-on: https://go-review.googlesource.com/c/go/+/171151
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>