Make the linker return an error status if it fails to merge ARM binaries with

Make the linker return an error status if it fails to merge ARM binaries with
different architecture tags.  Add a test case to make sure that this works,
and update readelf so that it will not seg-fault when trying to display the
attributes of binaries with invalid architecture tags.

	* elf32-arm.c (elf32_arm_merge_eabi_attributes): Return false if
	failed to merge.

	* ld-arm/attr-merge-arch-2.d: New test case.
	* ld-arm/attr-merge-arch-2a.s: New test case source file.
	* ld-arm/attr-merge-arch-2b.s: Likewise.
	* ld-arm/arm-elf.exp: Run new test case.

	* readelf.c (display_arm_attribute): Use unsigned int type for
	tag, val and type variables.