mono6: update to mono 6.4.0.198

- Use python tool.mk instead of searching more files to patch
@PYTHONBIN@ in.
- Add some preliminary ideas for how to get netbsd/aarch64 and solaris
working. it shouldn't be enough to complete a build.
(They can't use Mono's outdated libgc)

notable for us, this release re-adds FreeBSD supports.

Mono 6.4.0 release notes:
Highlights

    C# compiler support for C# 8 language version
    .NET Standard 2.1 support
    Updated libgdiplus to 6.0.2
    Notarized macOS installer package

In Depth
Runtime
Hardened Runtime and Notarization support on macOS

The Mono binary installed by the .pkg for macOS is now using the Hardened Runtime capabilities and the package was notarized to comply with Apple窶冱 new restrictions: https://developer.apple.com/documentation/security/notarizing_your_app_before_distribution.

This allows the package to work on the upcoming macOS 10.15 Catalina without showing warning dialogs.
Interpreter improvements

The Mono interpreter was updated to support the Windows operating system.

We also completed a lot of groundwork for upcoming future optimizations in the interpreter, like constant folding.
Bitness independent AOT cross compiler

The Ahead-Of-Time (AOT) cross compiler was updated to no longer require being executed with the same bitness that it should generate code for. This means a 64bit Mono can now emit AOT code for 32bit targets.

This work was mainly done to support executing the AOT cross compiler on macOS 10.15 Catalina (which is 64bit only) as we still need to generate code for 32bit targets like older iPhone and Apple Watch devices.
WebAssembly

We continue to work on making our WebAssembly support better. Various sets of issues with the debugger have been resolved in this release and general performance and feature work is happening as well.
LLVM improvements

We now leave it up to the LLVM framework to insert safepoints. Later optimizations can understand safepoints then which leads to better generated code. See https://github.com/mono/mono/pull/11789

The LLVM backend is also supported on the Windows operating system now.
PPC JIT optimizations

The PowerPC JIT received a bunch of optimization from community contributor Calvin Buckley (@NattyNarwhal).
Experimental build support for Fuchsia

A very minimal and experimental support for building Mono targeting the Fuchsia OS landed in the build system.
Class Libraries
.NET Standard 2.1 support

We updated our class libraries to support the latest additions to .NET Standard. You can now run a library compiled against the .NET Standard 2.1 specification on Mono.
CoreFX integration

We continued to replace some of our classes with the implementation from CoreFX to improve performance and compatibility with .NET.
libgdiplus update to 6.0.2

The libgdiplus native library is used for implementing System.Drawing on Unix platforms. This release contains many important improvements from our community members.

Special thanks go to Hugh Bellamy (@hughbe), Frederik Carlier (@qmfrederik) and Filip Navara (@filipnavara) for their awesome contributions!
System.Windows.Forms

More fixes and layout improvements for different controls made by external contributors have landed in this release .
Tools
C# 8 language version support in csc and msbuild

The C# compiler and msbuild tooling were updated to versions that support the final C# 8 language specification.

The Default Interface Methods (DIM) feature also received a few runtime enhancements.
NuGet

Bundled NuGet version has been upgraded to 5.2 RTM.

(maya)