libc: fix needless malloc failures

The NetBSD libc malloc implementation performs its own out-of-memory
check, presumably for performance reasons.  The check makes a strong
assumption about the address space layout, which is that memory-
mapped pages are always located above the heap.  However, this
assumption does not necessarily hold on MINIX3, thus resulting in
malloc reporting an out-of-memory condition without the system
actually being out of memory at all.  Evidence suggests that in
particular dynamically linked (i.e., pkgsrc) binaries were affected
by this issue - most notably git.

Change-Id: If542fbace0a1cce12aa9e075d51992cbbbf26e94

lib/libc/stdlib/malloc.c

@@ -818,8 +818,10 @@ imalloc(size_t size)
 
     if ((size + malloc_pagesize) < size)	/* Check for overflow */
 	result = NULL;
+#ifndef __minix
     else if ((size + malloc_pagesize) >= (uintptr_t)page_dir)
 	result = NULL;
+#endif /* !__minix */
     else if (size <= malloc_maxsize)
 	result = malloc_bytes(size);
     else