sparc32: fix PAGE_SIZE definition

Use _AC() in definition of PAGE_SIZE so the same definition
can be used in C and assembler.
Also use PAGE_SIZE in definition of THREAD_SIZE.

This commit kill the following comment:

    "I have my suspicions... -DaveM"

I did not find any clue what this referred to anyway.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>