powerpc/32s: Fix segments setup when TASK_SIZE is not a multiple of 256M

powerpc/32s: Fix segments setup when TASK_SIZE is not a multiple of 256M

For book3s/32 it is assumed that TASK_SIZE is a multiple of 256 Mbytes,
but Kconfig allows any value for TASK_SIZE.

In all relevant calculations, align TASK_SIZE to the upper 256 Mbytes
boundary.

Also use ASM_CONST() in the definition of TASK_SIZE to ensure it is
seen as an unsigned constant.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/8928d906079e156c59794c41e826a684eaaaebb4.1766574657.git.chleroy@kernel.org