Simplify amd64 trap().

Simplify amd64 trap().

- Use more relevant name 'signo' instead of 'i' for the local variable
  which contains a signal number to send for the current exception.
- Eliminate two labels 'userout' and 'out' which point to the very end
  of the trap() function.  Instead use return directly.
- Re-indent the prot_fault_translation block by reducing if() nesting.
- Some more monor style changes.

Requested and reviewed by:	bde
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week