powerpc/booke64: Add crit/mc/debug support to EXCEPTION_COMMON

Use the proper scratch SPRG and PACA region.  Introduce level-specific
macros to simplify usage and avoid needing to do a bunch of token
pasting throughout EXCEPTION_COMMON().

Now that EXCEPTION_COMMON_DBG() is properly using the debug scratch
register, there's no more need for the caller to move the value to the
GEN scratch first.

Signed-off-by: Scott Wood <scottwood@freescale.com>