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

pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go