Reduce #ifs by weeding out some cross-chunk function definitions.

Reduce #ifs by weeding out some cross-chunk function definitions.

Reduce the need for placing #if guards around functions that are
marked as VISIBLE in another code chunk by declaring the functions as
"weak" when they are not needed.

It's still necessary to ensure that no C code references the data from
a different chunk (or an -fwhole-program compile might try to keep a
local reference).