core: Implement 32-bit mode

core: Implement 32-bit mode

In 32-bit mode, effective addresses are truncated to 32 bits, both for
instruction fetches and data accesses, and CR0 is set for Rc=1 (record
form) instructions based on the lower 32 bits of the result rather
than all 64 bits.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>