[ARC] Fix case-sensitivity for extension instructions.

[ARC] Fix case-sensitivity for extension instructions.

In ARC assembler, we accept case insensitive mnemonics, but this was
not the case for extension instruction, fix it and add a test.

gas/
Claudiu Zissulescu <claziss@synopsys.com>

	* config/tc-arc.c (tokenize_extinsn): Convert to lower case the
	name of extension instructions.
	* testsuite/gas/arc/textinsn_case.d: New file.
	* testsuite/gas/arc/textinsn_case.s: Likewise.