Support standalone, segment, and REX prefixes in GAS mode.

Support standalone, segment, and REX prefixes in GAS mode.
* bytecode.c (yasm_bc_create_empty_insn): New function to create empty
instruction that can have prefixes applied to it, for standalone prefixes.
* bytecode.h (yasm_bc_create_empty_insn): Prototype.

* x86arch.h (x86_parse_insn_prefix): Add prefix types for segment registers
(X86_SEGREG) and REX bytes (X86_REX).
(yasm_x86__bc_apply_prefixes): Adjust prototype to include REX pointer (as
this isn't in the x86_common structure).
* x86bc.c (yasm_x86__bc_apply_prefixes): Support the new prefix types.

* x86id.re (x86_finalize_*): Use const x86_insn_info; all insn_infos are
const so these pointers should be as well.
(yasm_x86__finalize_insn): Handle empty instruction case by pointing to new
empty_insn info.
(empty_insn): New.
(yasm_x86__parse_check_prefix): Support GAS prefix naming, and REX and jump
hint prefixes (only in GAS mode at the moment).

* gas-bison.y: Add rules to handle segreg prefixes as well as standalone
prefixes (both segreg and others).

* gas-prefix.asm: New testcase that also hits the warning cases in
yasm_x86__bc_apply_prefixes X86_REX case.

svn path=/trunk/yasm/; revision=1292