test/run: limit parallelism to 1 for cross-exec builds
test/run: limit parallelism to 1 for cross-exec builds

This matters for NaCl, which seems to swamp my 4-core MacBook Pro otherwise.
It's not a correctness problem, just a usability problem.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/98600046

d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh