powerpc/32: Handle bookE debugging in C in exception entry

The handling of SPRN_DBCR0 and other registers can easily
be done in C instead of ASM.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6d6b2497115890b90cfa72a2b3ab1da5f78123c2.1615552866.git.christophe.leroy@csgroup.eu