[mingw] Use unmangled name after the $ in the section name

GCC does it this way, and we have to be consistent. This includes
stdcall and fastcall functions with suffixes. I confirmed that a
fastcall function named "foo" ends up in ".text$foo", not
".text$@foo@8".

Based on a patch by Andrew Yohn!

Fixes PR39218.

Differential Revision: https://reviews.llvm.org/D54762

llvm-svn: 347431