linux: pass --as-needed to linker

Gentoo has some nice docs on it here:
  http://www.gentoo.org/proj/en/qa/asneeded.xml
on my Maverick box I couldn't see a difference in the ldd output,
but I suppose it can't hurt on older systems.

BUG=11595

Review URL: http://codereview.chromium.org/3050043

git-svn-id: http://src.chromium.org/svn/trunk/src/build@55302 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -951,6 +951,7 @@
               # Specifically tell the linker to perform optimizations.
               # See http://lwn.net/Articles/192624/ .
               '-Wl,-O1',
+              '-Wl,--as-needed',
             ],
             'conditions' : [
               ['no_gc_sections==0', {