gccrs: Add array length to the proc macro buffer

gccrs: Add array length to the proc macro buffer

The compiler cannot infer the array length from the type, we should
therefore hand it the information. The proc macro buffer missed that
information.

gcc/rust/ChangeLog:

	* backend/rust-compile.cc (proc_macro_buffer): Update type builder with
	array length information.
	(proc_macro_array): Update type initializer with array length
	information.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>