xtensa: merge crt1-*.S

We had two assembly files to prepare for entry into C domain,
one intended for the simulator and one intended for real boards.

- Both files merged into a single crt1.S for either simulated or real
  targets
- Extra logic to populate command line arguments from simulator removed,
  we don't use it.
- BSS zeroing logic from crt1-boards.S used
- Reference to missing reset-unneeded.S removed
- exit() implementation moved to fatal.c, now invokes a kernel panic
  if we are not running under the simulator

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