[ELF] Use MaxPageSize for aligning PT_LOAD

This matches the behavior of Binutils linkers. We also change the
default MaxPageSize on x86-64 to 0x1000 to preserver the current
behavior, which is the same as the behavior implemented by gold.

https://llvm.org/bugs/show_bug.cgi?id=30541

Differential Revision: https://reviews.llvm.org/D24987

llvm-svn: 282560