Vanilla clang supports altmacro since clang 5.0, and thus doesn't
require gas-preprocessor for building the arm assembly any longer.

However, the built-in assembler doesn't support .dn directives.

This readds checks that were removed in d7320ca3, when
the last usage of .dn directives within libav were removed.

Alternatively, the assembly could be rewritten to not use the
.dn directive, making it available to clang users.
Signed-off-by: Martin Storsjö <martin@martin.st>