[ARC] Fix ARC target specific tests.

[ARC] Fix ARC target specific tests.

Fix ARC specific tests by improving the matching pattern and adding
the missing functionality in arc.exp

gcc/tests
xxxx-xx-xx  Claudiu Zissulescu  <claziss@synopsys.com>

	* gcc.target/arc/add_n-combine.c: Match add1/2/3 instruction in
	output assembly.
	* gcc.target/arc/arc.exp (check_effective_target_codedensity):
	Add.
	* gcc.target/arc/cmem-7.c: Fix matching patterns.
	* gcc.target/arc/cmem-bit-1.c: Likewise.
	* gcc.target/arc/cmem-bit-2.c: Likewise.
	* gcc.target/arc/cmem-bit-3.c: Likewise.
	* gcc.target/arc/cmem-bit-4.c: Likewise.
	* gcc.target/arc/interrupt-2.c: Match rtie insn for A7.
	* gcc.target/arc/store-merge-1.c: This test is only meaningful for
	architectures with double load/store operations.

From-SVN: r278609