gccrs: Add input/output from inout and split in out

gccrs: Add input/output from inout and split in out

Inline assembly was incomplete and input/output from inout or split in
out were not handled.

gcc/rust/ChangeLog:

	* backend/rust-compile-asm.cc (get_out_expr): Return valid output from
	an operand.
	(CompileAsm::asm_construct_outputs): Handle every output
	(get_in_expr): Return valid input from an operand.
	(CompileAsm::asm_construct_inputs): Handle every input

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