assemble.c: Don't drop rex prefix from instruction itself

assemble.c: Don't drop rex prefix from instruction itself

emit_rex is supposed to write REX prefix into output stream
if needed, but we happen to drop it off on a first write
which breaks REX required instructions if TIMES directive
is used.

For example the code like

	| times 4		movq	xmm11, xmm11

compiles into

	| 0000000000000000 <.text>:
	|   0:	f3 45 0f 7e db       	movq   %xmm11,%xmm11
	|   5:	f3 0f 7e db          	movq   %xmm3,%xmm3
	|   9:	f3 0f 7e db          	movq   %xmm3,%xmm3
	|   d:	f3 0f 7e db          	movq   %xmm3,%xmm3

instead of proper

	| 0000000000000000 <.text>:
	|   0:	f3 45 0f 7e db       	movq   %xmm11,%xmm11
	|   5:	f3 45 0f 7e db       	movq   %xmm11,%xmm11
	|   a:	f3 45 0f 7e db       	movq   %xmm11,%xmm11
	|   f:	f3 45 0f 7e db       	movq   %xmm11,%xmm11

http://bugzilla.nasm.us/show_bug.cgi?id=3392278

Reported-by: Javier <elpochodelagente@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>