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.

Initial sanity checks (pre-CI):

• common.h does not include any backend header.
• Backend headers are instead included in those *.c files where they are needed.
• ./scripts/tests all passes.