Default e_machine to EM_IAMCU for i?86-*-elfiamcu

Default e_machine to EM_IAMCU for i?86-*-elfiamcu

This patch sets the default ELF output format of assembler and linker to
EM_IAMCU when binutils is configured to i?86-*-elfiamcu target.

gas/

	* configure.tgt (arch): Set to iamcu for i386-*-elfiamcu target.
	* config/tc-i386.c (i386_mach): Support iamcu.
	(i386_target_format): Likewise.

ld/

	* configure.tgt: Support i[3-7]86-*-elfiamcu target.

ld/testsuite/

	* ld-i386/i386.exp (iamcu_tests): Run iamcu-4.
	* ld-i386/iamcu-4.d: New file.