[PECOFF] Set ordinals to linker internal atoms.

This patch won't change the output because the layout of linker internal
atoms is forced by layout-{before,after} references. Ordinals of the linker
internal atoms are not currently used. (That's why it's working even if there
are atoms having the same ordinals.)

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195610 91177308-0d34-0410-b5e6-96231b3b80d8