[ELF] - Align sections file offsets correctly.

System V ABI 4.1 specifies that program header's p_vaddr should equal p_offset, modulo p_align. 
(https://docs.oracle.com/cd/E19683-01/816-1386/chapter6-83432/index.html).
It was possible to violate this using the linkerscript.
Patch fixes the issue.

Differential revision: http://reviews.llvm.org/D19416

llvm-svn: 267684