[ELF] Initialize PhdrEntry::p_align to maxPageSize for PT_LOAD

```
Writer<ELFT>::run
  assignFileOffsets
    setFileOffset
      computeFileOffset
        os->ptLoad->p_align may be smaller than config->maxPageSize
  setPhdrs
    p_align = max(p_align, config->maxPageSize)
```

If we move the config->maxPageSize logic to the constructor of
PhdrEntry, computeFileOffset can be simplified.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D67211

llvm-svn: 371085