[MinGW] Support dllexport on i386

In MinGW configurations (GCC, or clang with a *-windows-gnu target),
the -export directives in the object file contains the undecorated
symbol name, while it is decorated in MSVC configurations. (On the
command line, link.exe takes an undecorated symbol name for the
-export argument though.)

Differential Revision: https://reviews.llvm.org/D37772

llvm-svn: 313174