cmd/internal/obj/arm: make assembler concurrency-safe
Move global state from obj.Link
to a new function-local state struct arm.ctxt5.
This ends up being cleaner than threading
all the state through as parameters; there's a lot of it.
While we're here, move newprog from a parameter to ctxt5.
We reserve the variable name c for ctxt5,
so a few local variables named c have been renamed.
Instead of lazily initializing deferreturn
and Sym_div and friends, initialize them up front.