Previously, LLD would always set the implicit entry point for DLLs to
the symbol that is prefixed with an underscore. However, mingw-w64
defines it without that underscore.
This change fixes that by adding a special branch for MinGW. Also, it
simplifies tests that use MinGW style DLL entry symbol by skipping the
entry point argument.
Note, tests that use MSVC style DLL entry symbol and LLD in MinGW mode,
will now require using explicit entry point. I believe this is sensible.
When an explicit entry point is passed, i.e. LLD is called by Clang or
GCC, there will be no observable difference.
Fixes https://github.com/llvm/llvm-project/issues/171441
2824225d27