Do not hard code the leading underscore.

Mangling scheme varies on platform, and prepending an underscore is valid only
on 32-bit x86. Added a method to mangle name to PECOFFLinkingContext and use
it to avoid hard coding mangled names.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190585 91177308-0d34-0410-b5e6-96231b3b80d8