Add macros for diagnostic control, use them in locale/weightwc.h

[Roland McGrath]

> Neither you nor Roland liked the IF_LINT approach that Gnulib uses for 
> this sort of thing, but I hope we can figure out something else to 
> satisfy glibc's constraints.  How about something like this?
> 
> #define ASSUME_MEM_INITIALIZED(v) asm ("" : "=m" (v))
> 
> and then write "ASSUME_MEM_INITIALIZED (resplen)" after declaring resplen.

With "=g" as the constraint it probably won't affect code generation at all.
So that could be OK.