build: add avresample after avcodec to FFLIBS

avcodec might depend on avresample and with --as-needed required symbols
might be get removed if avresample is linked before avcodec.
Fixes link failures of avprobe and avplay on aarch64 with
--enable-neon-clobber-test.
configure: fix enable-libopus help string
aarch64: opus NEON iMDCT and FFT

Opus celt decoding 11% faster and the iMDCT over 2.5 times faster on
Apple's A7.