GN: Use rsp file, not rsp file contents, in solink line

GN: Use rsp file, not rsp file contents, in solink line

Passing $rspfile_content passes the content of the rsp file verbatim
to the command line, which kind of defeats the purpose of using rsp files.
@\$rspfile tells the linker to open up the file itself.

BUG=398255
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=286675

[jamesr, 2014-07-29 23:52:52]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2014-07-29 23:57:26]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamesr@chromium.org/428013002/1

[commit-bot: I haz the power, 2014-07-29 23:57:27]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-07-29 23:57:27]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[brettw, 2014-07-30 22:22:27]

ooh, clever!

LGTM

[jamesr, 2014-07-30 22:32:04]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2014-07-30 22:33:19]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamesr@chromium.org/428013002/1

[commit-bot: I haz the power, 2014-07-31 01:33:35]

Change committed as 286675