[PECOFF] Fix __imp_ prefix on x64.

"__imp_" prefix always starts with double underscores.
When I was writing the original code I misunderstood
that it's "_imp_" on x64.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218690 91177308-0d34-0410-b5e6-96231b3b80d8