Add support for extensions in the .machine pseudoop on S/390, e.g. ".machine zEC12...

Add support for extensions in the .machine pseudoop on S/390, e.g. ".machine zEC12+nohtm+vx"

gas	* doc/c-s390.texi: Add documentation.
	Add missing code markup.
	* config/tc-s390.c (current_flags): New static variable.
	(s390_parse_cpu): Parse cpu flags a la "+nohtm" etc.
	(s390_setup_opcodes): Use cpu flags to determine the set of opcodes.
	Fix indentation.
	(md_parse_option): Call s390_parse_cpu with the new signature.
	(s390_machine): Likewise.
	Keep track of current_flags.
	Simplify code a bit.
	undefine MAX_HISTORY at end of function.
	(s390_machinemode): undefine MAX_HISTORY at end of function.
	Update an error message.

tests	* gas/s390/s390.exp: Add new tests.
	* gas/s390/machine-parsing-1.s: New test file.
	* gas/s390/machine-parsing-1.l: Likewise.
	* gas/s390/machine-parsing-2.s: Likewise.
	* gas/s390/machine-parsing-2.l: Likewise.
	* gas/s390/machine-parsing-3.s: Likewise.
	* gas/s390/machine-parsing-3.l: Likewise.
	* gas/s390/machine-parsing-4.s: Likewise.
	* gas/s390/machine-parsing-4.l: Likewise.
	* gas/s390/machine-parsing-5.s: Likewise.
	* gas/s390/machine-parsing-5.l: Likewise.
	* gas/s390/machine-parsing-6.s: Likewise.
	* gas/s390/machine-parsing-6.l: Likewise.

opcode	* s390.h (S390_INSTR_FLAG_HTM): New flag.
	(S390_INSTR_FLAG_VX): New flag.
	(S390_INSTR_FLAG_FACILITY_MASK): New flag mask.

opcodes	* s390-mkopc.c (main): Parse htm and vx flag.
	* s390-opc.txt: Mark instructions from the hardware transactional
	memory and vector facilities with the "htm"/"vx" flag.