gccrs: Static Items must be const evaluated

gccrs: Static Items must be const evaluated

Statics like constants need to have a singular value they are not functions
to be lazy evaluated. So to evaluate a block expr we can just reuse our
const code to resolve this to a singular value.

gcc/rust/ChangeLog:

	* backend/rust-compile-item.cc (CompileItem::visit): Const evaluate
	static item expressions.