Use PNaCl built binutils, which is known to support ARM and MIPS.
Otherwise the system-provided binutils may or may not have that
support (mine did not and perhaps expected a prefix like
arm-xxx-objcopy for the version that did support arm).

Split off from CL to run crosstests for ARM under qemu.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1185703006.