gccrs: Fix NR2 ICE in visit_attributes

gccrs: Fix NR2 ICE in visit_attributes

Undefined attribute macros have no proc macro definition, which results
in a failing `rust_assert`. This changes that assert to an if statement,
that returns early if there is no proc macro definition. Fixes #3661.

gcc/rust/ChangeLog:

	* resolve/rust-early-name-resolver-2.0.cc (Early::visit_attributes): rust_assert to if

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3661.rs: Test NR2 has expected behavior

Signed-off-by: Tom Schollenberger <tss2344@g.rit.edu>