linux: pass --gc-sections in link line

linux: pass --gc-sections in link line

We have compile flags set up to support using --gc-sections, but apparently
we aren't using it for the actual link!  It was probably lost during the gyp
conversion.

This has a few-megabyte difference on binary size.

(Trying submit again, hopefully thestig successfully converted bots...)

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=32330

[tony, 2009-11-17 23:25:30]

LG (evan tells me this saves about 4mb!)

[agl, 2009-11-17 23:25:32]

LGTM

--gc-sections has been enabled at some point in the past. Maybe it got lost when
switching to GYP.

(Also, how much of a size reduction do we get?)