Fix compilation errors.
vec_avx.h needs x86cpu.h, and x86cpu.h needs to detect SSE2 (or greater) without the Opus macros. Also, nobody was defining OPUS_CLEAR (but several things were including the non-existent os_support.h where it is defined in libopus), so replace those calls with RNN_CLEAR and remove the erroneous includes. Take the opportunity to hoist OPUS_GNUC_PREREQ to common.h, too, since it is needed in multiple places now.
Fixes GitHub #222