configure.in: check for `nasm` only on `i486`

NASM is useful only on x86 architecture. There it should be listed which architectures are supported by NASM and which are not.

Based on patch by Koen Kooi [1].

[1] http://cgit.openembedded.org/cgit.cgi/openembedded/commit/recipes/xbmc/xbmc/0001-Only-check-for-nasm-on-i686-it-is-bogus-on-ARM-which.patch?id=0cc02ded7bd429208ccb0e0e0aa3e43aec8a4722

Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Koen Kooi <koen@openembedded.org>
Closes: Trac ticket #11020