gas/arc: Unify tests run on big/little endian arc assembler

We were running a slightly different set of assembler tests on big and
little endian arc targets.  This commit unifies the set of tests run.

gas/ChangeLog:

	* testsuite/gas/arc/add_s-err.s: Update target pattern.
	* testsuite/gas/arc/warn.s: Likewise.
	* testsuite/gas/elf/elf.exp: Run test for arc.