powerpc/603: Avoid a pile of NOPs when not using SW LRU in TLB exceptions

The SW LRU is in an MMU feature section. When not used, that's a
dozen of NOPs to fetch for nothing.

Define an ALT section that does the few remaining operations.

That also avoids a double read on SRR1 in the SW LRU case.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/603725297466959419628ef7964aaf3417fb647d.1620363691.git.christophe.leroy@csgroup.eu