arm: Avoid using .dn register aliases

clang now (in the upcoming 5.0 version) is capable of building our
arm assembly without relying on gas-preprocessor, although clang/LLVM
doesn't support .dn register aliases.

The VC1 MC assembly was only built and used if the chosen assembler
supported the .dn directives though. This was supported as long as
gas-preprocessor was used.

This means that VC1 decoding got a speed regression on clang 5.0,
unless the user manually chose using gas-preprocessor again.

By avoiding using the .dn register aliases, we can build the VC1 MC
assembly with the latest clang version.

Support for the .dn/.qn directives in clang/LLVM isn't actively planned,
see https://bugs.llvm.org/show_bug.cgi?id=18199

.

This partially reverts 896a5bff.

Signed-off-by: Martin Storsjö <martin@martin.st>