gccrs: libproc_macro: Change constructor in ffistring

The "new" constructor wasn't fitting it's usage well.

libgrust/ChangeLog:

	* libproc_macro/rust/bridge/ffistring.rs: Implement
	From trait for FFIString.
	* libproc_macro/rust/bridge/literal.rs: Change
	constructor call.

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