bsd-user/mmap.c: Don't try to override g_malloc/g_free

Trying to override the implementations of g_malloc and g_free is
a really bad idea -- it means statically linked builds fail to
link (because of the multiple definitions provided by this file
and by glib), and non-statically linked builds segfault as soon
as they try to do anything more complicated than printing the
usage message. Remove these overridden versions and just use
the glib ones.

This is sufficient that bsd-user can run basic x86-64
binaries on OpenBSD again; FreeBSD and NetBSD seem to have
further issues.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sean Bruno <sbruno@freebsd.org>
Reviewed-by: Ed Maste <emaste@freebsd.org>
