bug#22086: 25.1.50; [PATCH] Integrate the musl hybrid malloc patch for elf systems

[Paul Eggert <eggert@cs.ucla.edu>]

While thinking over this patch I'd like to propose what should be a simpler 
approach. This new proposal is more radical, and so should not be applied to the 
emacs-25 branch, but it should make the port to musl etc. automatic.

The simpler approach is to remove gmalloc.c, and to use the system memory 
allocator, i.e., to behave as if SYSTEM_MALLOC is defined on all platforms.

We can still support hybrid malloc for Cygwin, if SYSTEM_MALLOC wouldn't work on 
Cygwin for some reason; and we can support the similar hybrid on Darwin, if it's 
still needed. But in neither approach should we override the system malloc; any 
Emacs-specific allocation function we define should be called (say) emalloc 
instead of malloc, so that it does not conflict with the system malloc. That 
way, we don't have to worry about name-space collisions, either at compile-time 
or at link-time.

If I'm wrong about gmalloc.c and it is still needed on some platforms for some 
reason, we can continue to use it, but it should define emalloc etc., and not 
attempt to override the C standard library.

Long ago as I recall, we really needed to override the C standard library on 
some platforms, due to the funny way in which undumped storage was made 
read-only. That need is obsolete, though, which should let us simplify things now.