rust: enable Control Flow Guard on gnullvm

[mati865]

It works but not sure whether we should do it.

[ognevny]

for a future reference, why can't we enable it for other environments? it depends on support from local mingw-w64 runtime, right?

[mati865]

I didn't test it, but I suppose the whole toolchain has to support it.

[lazka]

Just to clarify, does this enable it

• for building rust itself?
• for things built with rustc?
• for users to enable it with rustc?

and are there any differences to the official upstream builds?

[mati865]

for building rust itself?

It's enabled for libstd (equivalent of libstdc++ from GCC), that is used by Rust and things built with Rust. The comment mentions only libstd, but I think it might be enabled for the compiler as well.

for things built with rustc?

Not explicitly, it's similar to mingw-w64. Some checks will be pulled from the library, but to fully benefit from the feature (at a small performance penalty), it has to be enabled manually.

for users to enable it with rustc?

It allows enabling that option.

and are there any differences to the official upstream builds?

AFAICT upstream doesn't enable it in any provided builds.