configure: arm: do not enable fast-unaligned if it is explicitly disabled via commandline

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Fix build with --enable-ladspa --disable-frei0r.

https://bugs.gentoo.org/show_bug.cgi?id=510516

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>