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.