[MIPS] Update SConstruct to use new MIPS toolchain.

[MIPS] Update SConstruct to use new MIPS toolchain.

Recent switch to a newer 4.7.2 GCC for MIPS requires changes in SConstruct.
The change also includes minor change to comply with unused-but-set-variable
restriction.

The commit queue is not working for NaCl at the moment, so we are committing
with:
NOTRY=true
However, we have run a try job manually, and it passed fine.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=2275
TEST= pnacl/build.sh misc-tools

Committed: http://src.chromium.org/viewvc/native_client?view=rev&revision=10009

[petarj, 2012-10-16 00:11:07]

This is a necessary follow to CL in which we changed the toolchain for
MIPS: https://codereview.chromium.org/11036032/

[Mark Seaborn, 2012-10-16 01:11:37]

LGTM.  I ran into these problems today, as it happens.  Thanks for fixing them
before I mentioned them! :-)

[commit-bot: I haz the power, 2012-10-16 09:02:05]

CQ is trying da patch. Follow status at
https://nativeclient-status.appspot.com/cq/petarj@mips.com/11151031/1

[commit-bot: I haz the power, 2012-10-16 10:04:02]

The commit queue went berserk retrying too often for a
seemingly flaky test. Builder is nacl-arm_opt_panda, revision is 10006, job name
was 11151031-1 (previous was lost) (previous was lost) (previous was lost)
(previous was lost).

[commit-bot: I haz the power, 2012-10-16 11:01:56]

CQ is trying da patch. Follow status at
https://nativeclient-status.appspot.com/cq/petarj@mips.com/11151031/1

[commit-bot: I haz the power, 2012-10-16 11:02:01]

Change committed as 10009