Restore proper dot symbol creation for assembly files in the kernel build case.

Restore proper dot symbol creation for assembly files in the kernel build case.
Without this patch we were not able to see the assembly function.
Only the function descriptor was visible.

- Distinguish between user-land and kernel when creating the ENTRY() point of
  assembly source.
- Make the ENTRY() macro more readable, replace the .align directive with the
  gas platform independant .p2align directive.
- Create an END()macro for later use to provide traceback tables on powerpc64.