nios2: crt0: fix GDB debugging

When an image is sent over the wire with the GDB 'load' command,
it tries to start execution from the __start symbol, which needs to
be in RAM. Since the reset vector is in ROM, name it something else.

Change-Id: Id0bbfa76db9a8a81bd7ff20be3f2baec81eae15e
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>