ELF: Align RELRO to the target page size rather than the max page size.

If no bss sections appear after the relro segment, the loader will round
the r/w segment size to the target's page size. Align the relro size in the
same way to ensure that it does not extend past the end of the program's
own memory region.

Differential Revision: https://reviews.llvm.org/D29242

llvm-svn: 293519