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)