Always define gmalloc etc. in src/gmalloc.c

Always define gmalloc etc. in src/gmalloc.c

This is a work-around to prevent the compiler from using semantic
knowledge about malloc for optimization purposes.  E.g., gcc 5.2
with -O2 replaces most of calloc's definition by a call to calloc;
see Bug#22085.
* src/gmalloc.c [!HYBRID_MALLOC] (malloc, realloc, calloc)
(aligned_alloc, free): Do not undef.  Instead, define these as
functions (perhaps renamed to gmalloc etc.) in terms of gmalloc etc.