x86: Accept Intel64 only instruction by default

x86: Accept Intel64 only instruction by default

Commit d835a58baae720 disabled sysenter/sysenter in 64-bit mode by
default.  By default, assembler should accept common, Intel64 only
and AMD64 ISAs since there are no conflicts.

gas/

	PR gas/25516
	* config/tc-i386.c (intel64): Renamed to ...
	(isa64): This.
	(match_template): Accept Intel64 only instruction by default.
	(i386_displacement): Updated.
	(md_parse_option): Updated.
	* c-i386.texi: Update -mamd64/-mintel64 documentation.
	* testsuite/gas/i386/i386.exp: Run x86-64-sysenter.  Pass
	-mamd64 to x86-64-sysenter-amd.
	* testsuite/gas/i386/x86-64-sysenter.d: New file.

opcodes/

	PR gas/25516
	* i386-gen.c (opcode_modifiers): Replace AMD64 and Intel64
	with ISA64.
	* i386-opc.h (AMD64): Removed.
	(Intel64): Likewose.
	(AMD64): New.
	(INTEL64): Likewise.
	(INTEL64ONLY): Likewise.
	(i386_opcode_modifier): Replace amd64 and intel64 with isa64.
	* i386-opc.tbl (Amd64): New.
	(Intel64): Likewise.
	(Intel64Only): Likewise.
	Replace AMD64 with Amd64.  Update sysenter/sysenter with
	Cpu64 and Intel64Only.  Remove AMD64 from sysenter/sysenter.
	* i386-tbl.h: Regenerated.

(cherry picked from commit 4b5aaf5f6992319c2c72e080a1a55842640b8732)