[PECOFF] Fix atom default alignment.

Alignment(1) does not mean that the atom should be aligned on a 1 byte
boundary but on a 2^1 boundary. So, atoms without any specific alignment
requirements should have Alignment(0).

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190723 91177308-0d34-0410-b5e6-96231b3b80d8