nios2: crt0: split into __start and __text_start

__start is the CPU's reset vector. In a typical Nios II configuration,
the exception vector is 0x20 bytes after the reset vector, severely
constraining the amount of code that can go in here.

Split this into __start and __text_start. The only thing that __start
really need to do is init the instruction cache and jump into
__text_start.

JIRA IDs placed in comments for missing items.

Change-Id: I3c6b8ed65e8fcf6b6a735b80cf007d0180599230
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>