ARM: make CRT_LEGACY_WORKAROUND work as intended

To properly support legacy ARM shared libraries, libc.so needs
to export the symbols __dso_handle and atexit, even though
these are now supplied by the crt startup code.

This patch reshuffles the existing CRT_LEGACY_WORKAROUND
conditionally compiled code slightly so it works as the
original author likely intended.

Change-Id: Id6c0e94dc65b7928324a5f0bad7eba6eb2f464b9
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@gmail.com>