gccrs: Add a function to check if a function is variadic

gccrs: Add a function to check if a function is variadic

This function provides an easy way to check for a function's varidicity.

gcc/rust/ChangeLog:

	* ast/rust-item.h: Add a getter to check if a given function is
	variadic.

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