[POWERPC] Move the exception macros into a header file

It makes head_64.S a bit more readable and will allow us to move the
iSeries exceptions elsewhere.

This also removes the last line of the comment:

 * The following macros define the code that appears as
 * the prologue to each of the exception handlers.  They
 * are split into two parts to allow a single kernel binary
 * to be used for pSeries and iSeries.
 * LOL.  One day... - paulus

Anything is possible. :-)
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>