Bug 1512723 - Enable cross-language LTO on Win64 shippable builds r=glandium

Differential Revision: https://phabricator.services.mozilla.com/D25852

--HG--
extra : moz-landing-system : lando

browser/config/mozconfigs/win64/common-opt

@@ -20,3 +20,7 @@ export MOZ_TELEMETRY_REPORTING=1
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1
+
+if [ -n "$MOZ_PGO" ]; then
+    RUSTFLAGS="-Clinker-plugin-lto"
+fi