gccrs: Fix ICE using constructors for intilizers in statics

gccrs: Fix ICE using constructors for intilizers in statics

We are getting constant expressions for the initilizers for static items
this hits an assertion in the GCC middle-end which is looking for a
constructor so we need to unwrap the constant expression using DECL_INITIAL
as the initilizer to the global static.

Fixes #2080

gcc/rust/ChangeLog:

	* backend/rust-compile-item.cc (CompileItem::visit): unwrap the constant expression

gcc/testsuite/ChangeLog:

	* rust/execute/torture/issue-2080.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>