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...)

Review URL: http://codereview.chromium.org/399048

git-svn-id: http://src.chromium.org/svn/trunk/src/build@32330 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -618,6 +618,9 @@
               '-fdata-sections',
               '-ffunction-sections',
             ],
+            'ldflags': [
+              '-Wl,--gc-sections',
+            ],
           },
         },
         'variants': {