Add a dummy atexit(3) to the linker.

This is needed when building the linker against a libc that was built
with coverage information.

Bug: 17553780
Change-Id: If131002e067dd7768fc657c30e62c42a45c05ace

linker/linker_libc_support.c

@@ -15,3 +15,7 @@
  */
 
 #include "../libc/arch-common/bionic/__dso_handle.h"
+
+int atexit(void (*function)(void) __attribute__((__unused__))) {
+  return -1;
+}