* gas/config/tc-arm.c (md_pseduo_table): Add .arch_extension directive.

	* gas/config/tc-arm.c (md_pseduo_table): Add .arch_extension directive.
	(arm_option_extension_value_table): Add.
	(arm_extensions): Change type.
	(arm_option_cpu_table): Rename...
	(arm_option_fpu_table): ...to this.
	(arm_fpus): Change type.
	(arm_parse_extension): Enforce alphabetical order.  Allow
	extensions to be removed.
	(arm_parse_arch): Allow extensions to be specified with -march.
	(s_arm_arch_extension): Add.
	(s_arm_fpu): Update for type changes.
	* gas/doc/c-arm.texi: Document changes to infrastructure.