linux: migrate linker --threads flag out of gyp

(Retry of r121148 with ChromeOS excluded.)

This just copies the flags used within gyp.

BUG=gyp:250

Review URL: http://codereview.chromium.org/9360032

git-svn-id: http://src.chromium.org/svn/trunk/src/build@121257 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -389,13 +389,18 @@
 
         # linux_use_gold_binary: whether to use the binary checked into
         # third_party/gold.
-        # linux_use_gold_flags: whether to use build flags that rely on gold.
-        # On by default for x64 Linux.
         ['host_arch=="x64"', {
           'linux_use_gold_binary%': 1,
-          'linux_use_gold_flags%': 1,
         }, {
           'linux_use_gold_binary%': 0,
+        }],
+
+        # linux_use_gold_flags: whether to use build flags that rely on gold.
+        # On by default for x64 Linux.  Temporarily off for ChromeOS as
+        # it failed on a buildbot.
+        ['host_arch=="x64" and chromeos==0', {
+          'linux_use_gold_flags%': 1,
+        }, {
           'linux_use_gold_flags%': 0,
         }],
 
@@ -2003,6 +2008,13 @@
             'cflags': ['-finstrument-functions'],
           }],
           ['linux_use_gold_flags==1', {
+            'ldflags': [
+              # Experimentation found that using four linking threads
+              # saved ~20% of link time.
+              # https://groups.google.com/a/chromium.org/group/chromium-dev/browse_thread/thread/281527606915bb36
+              '-Wl,--threads',
+              '-Wl,--thread-count=4',
+            ],
             'conditions': [
               ['release_valgrind_build==0', {
                 'target_conditions': [