[ELF2] Make the system page size a target-dependent property

The required page alignment is different on different targets. On PowerPC, for
example, we need 64K pages (the loader won't set different permissions on a
finer granularity than that). I've set the existing targets to what I believe
to be the correct values, and have updated the regression tests accordingly.

llvm-svn: 249760
7 files changed