compiler: avoid GCC middle-end control warnings

compiler: avoid GCC middle-end control warnings

    GCC has started emitting "control reaches end of non-void function"
    warnings. Avoid them for Go by 1) marking the builtin function panic
    and the compiler-generated function __go_runtime_error as not
    returning and 2) adding a default case to the switch used for select
    statements that simply calls __builtin_unreachable.

    Fixes golang/go#22767

    Reviewed-on: https://go-review.googlesource.com/80416

	* go-gcc.cc (Gcc_backend::Gcc_backend): Define
	__builtin_unreachable.
	(Gcc_backend::function): Add does_not_return parameter.

From-SVN: r255346