arm: check if AS supports .dn
Move the GNU as check before the arch specific asm checks since the .dn check requires gas compatible assembler. Disable the VC-1 motion compensation NEON asm which is the only part using that directive. The integrated assembler in the upcoming clang 3.5 does not support .dn/.qn without plans to change that. Too much effort to implement it while it is rarely used. http://llvm.org/bugs/show_bug.cgi?id=18199.
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