Compute GC_CONS arguments just once

Compute GC_CONS arguments just once
(fix of commit 8eb6f8d)

* include/gc_inline.h (GC_CONS): Declare l and r local variables;
compute first and second expression even in case of
GC_MALLOC_WORDS_KIND failure; pass l and r to GC_reachable_here (instead
of first and second).