arm: fix extension feature disabling

arm: fix extension feature disabling

Using e.g.

	.arch_extension simd
	.arch_extension nocrypto

so far results in SIMD support getting disabled, which I can't see being
the purpose of the "no"-prefixed variants of architecture extension
specifications.

Of course it is questionable whether the current, counter intuitive
behavior needs to be retained, and the new behavior perhaps be made work
through e.g. a newly recognized "no-" prefix.

gas/
2015-01-07  Jan Beulich <jbeulich@suse.com>

	* gas/config/tc-arm.c (struct arm_option_extension_value_table):
	Split field "value" into fields "merge_value" and "clear_value".
	(arm_extensions): Adjust initializer accordingly.