Remove backend header import from common.h #1182
Description
Currently, common.h includes the backend headers with the static inline wrappers around backend functionality. For some olders compilers (gcc-4.8) in -O0 this leads to linker failures if the backend itself is not linked. This makes it hard to test files in isolation -- specifically, in #1135 I'm struggling to create a build which includes specific backend assembly files without including the entire backends (including twiddle tables).
Consider removing the backend header import from common.h, and instead import it manually where needed.