Fix the "opt-win" build on Visual Studio 10.0

Fix the "opt-win" build on Visual Studio 10.0

Before the change, linking fails with:
LINK : fatal error LNK1117: syntax error in option 'PDBPATH:none'

"/PDBALTPATH:%_PDB%" works on both MSVC 9.0 and 10.0 and has the same
effect of omitting the absolute pathname from the executable.

Both before and after, we get:
$ strings scons-out/opt-win-x86-32/staging/ncdecode_table.exe | grep -i pdb
ncdecode_table.pdb

BUG=http://code.google.com/p/nativeclient/issues/detail?id=1287
TEST=build with Windows SDK + trybots

Committed: http://src.chromium.org/viewvc/native_client?view=rev&revision=4077

[Mark Seaborn, 2011-01-05 21:32:51]

I got the fix from https://jira.secondlife.com/browse/SNOW-728?

[Mark Seaborn, 2011-01-05 21:34:11]

On 2011/01/05 21:32:51, Mark Seaborn wrote:
> I got the fix from https://jira.secondlife.com/browse/SNOW-728?

Oops.  Wrong review.  See http://msdn.microsoft.com/en-us/library/dd998269.aspx
and http://msdn.microsoft.com/en-us/library/44wx0fef.aspx for the docs for these
PDBPATH options.

[bradn, 2011-01-05 21:36:08]

LGTM