[COFF] Fix the name mangling of a function in the autoexport exclusion list
[COFF] Fix the name mangling of a function in the autoexport exclusion list

The __NULL_IMPORT_DESCRIPTOR symbol has two leading underscores on
architectures other than i386 as well; it is not a mangled symbol name.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@342448 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld