runtime/internal/sys.Ctz32/64 and Bswap32/64 are currently
    implemented with compiler builtin functions. But if they are
    called from another package, the compiler does not know and
    therefore cannot turn them into compiler intrinsics. This CL
    makes the compiler recognize these functions and turn them into
    intrinsics directly, as the gc compiler does.
    
    This CL sets up a way for adding intrinsics in the compiler.
    More intrinsics will be added in later CLs.
    
    Also move the handling of runtime.getcallerpc/sp to the new way
    of generating intrinsics.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/176917

From-SVN: r271303