[dev.power64] runtime: test CAS on large unsigned 32-bit numbers
[dev.power64] runtime: test CAS on large unsigned 32-bit numbers

This adds a test to runtime?check to ensure CAS of large
unsigned 32-bit numbers does not accidentally sign-extend its
arguments.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/162490044

d0f/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh