This stubs out all of the PxSEGADDR() wrappers for non-legacy code.
29-bit will continue to work with these, while 32-bit code will now blow
up on compile rather than at runtime.

The vast majority of the in-tree offenders are gone, with the only
remaining culprits being unable to support 32-bit mode.

Hopefully this will prevent anyone from ever using these again.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>