nios2: add minimal crt0.S

At the moment this just jumps into prep_c, with comments left
on other things that need to be done. Having this here ensures that
the early boot code isn't discarded by gc-sections.

vector_table.c removed, it isn't the right approach for this CPU.
Proper method for initializing reset and exception vectors still
being investigated.

Change-Id: Id7965c671f1a55c42ecfb65119497405a646bec4
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>