[ELF] Fix the fileoffset according to the ELF spec.

With --no-align-segments, there is a bug that the fileoffset may not be
congruent to virtual address modulo page alignment.

This patch fixes the problem.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221890 91177308-0d34-0410-b5e6-96231b3b80d8