Tighten arm32/arm64 write barrier kill reg sets

Tighten arm32/arm64 write barrier kill reg sets

The JIT write barrier helpers have a custom calling convention that
avoids killing most registers. The JIT was not taking advantage of
this, and thus was killing unnecessary registers when a write barrier
was necessary. In particular, some integer callee-trash registers
are unaffected by the write barriers, and no floating-point register
is affected.

Also, I got rid of the `FEATURE_WRITE_BARRIER` define, which is always
set. I also put some code under `LEGACY_BACKEND` for easier cleanup
later. I removed some unused defines in target.h for some platforms.