compiler: intrinsify some math/bits functions

compiler: intrinsify some math/bits functions

    Let the Go frontend recognize some math/bits functions and turn
    them into intrinsics.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/183266

	* go-gcc.cc (Gcc_backend::Gcc_backend): Define math/bits
	builtins.

From-SVN: r272579