[PECOFF] Fix x64 export table.

Export table entry is 64 bit wide in x64. If MSB is 1, it means it's
imported by ordinal. The shift value was wrong.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218728 91177308-0d34-0410-b5e6-96231b3b80d8