[llvm-dlltool] Write correct weak externals

Previously, the created object files for the import library were broken.
Write the symbol table before the string table. Simplify the code by
using a separate variable Prefix instead of duplicating a few lines.

Also update the coff-weak-exports to actually check that the generated
weak symbols can be found as intended.

Differential Revision: https://reviews.llvm.org/D36065

llvm-svn: 309555