Split Alignment out of the Section Characteristics.

The alignment is just a byte in the middle of Characteristics, not an
independent flag. Making it an independent field in the yaml
representation makes it more yamlio friendly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181243 91177308-0d34-0410-b5e6-96231b3b80d8