Add Intel MCU support to gas

Add Intel MCU support to gas

-march=iamcu must be passed to i386 assembler to generate Intel MCU object
file.

gas/

	* config/tc-i386.c (cpu_arch): Add iamcu.
	(i386_align_code): Handle PROCESSOR_IAMCU.
	(i386_arch): Likewise.
	(i386_mach): Likewise.
	(i386_target_format): Likewise.
	(valid_iamcu_cpu_flags): New function.
	(check_cpu_arch_compatible): Only allow Intel MCU instructions
	when targeting Intel MCU.
	(set_cpu_arch): Call valid_iamcu_cpu_flags to check if CPU flags
	are valid for Intel MCU.
	(md_parse_option): Likewise.
	* tc-i386.h (ELF_TARGET_IAMCU_FORMAT): New.
	(processor_type): Add PROCESSOR_IAMCU.
	* doc/c-i386.texi: Document iamcu.

gas/testsuite/

	* gas/i386/i386.exp: Run iamcu-1, iamcu-2, iamcu-3, iamcu-inval-1,
	iamcu-inval-2 and iamcu-inval-3.
	* gas/i386/iamcu-1.d: New file.
	* gas/i386/iamcu-1.s: Likewise.
	* gas/i386/iamcu-2.d: Likewise.
	* gas/i386/iamcu-2.s: Likewise.
	* gas/i386/iamcu-3.d: Likewise.
	* gas/i386/iamcu-3.s: Likewise.
	* gas/i386/iamcu-inval-1.l: Likewise.
	* gas/i386/iamcu-inval-1.s: Likewise.
	* gas/i386/iamcu-inval-2.l: Likewise.
	* gas/i386/iamcu-inval-2.s: Likewise.
	* gas/i386/iamcu-inval-3.l: Likewise.
	* gas/i386/iamcu-inval-3.s: Likewise.