GC_scratch_alloc code refactoring (and WARN message improvement)

* headers.c (GC_scratch_alloc): Remove "register" keyword for "result"
local variable; replace tail recursion with a loop; adjust and add
comments.
* headers.c (GC_scratch_alloc): Adjust message and output requested
allocation size in WARN().
* include/private/gc_priv.h (_GC_arrays._scratch_last_end_ptr):
Improve comment.