build/toolchain/gcc_toolchain.gni: further fixes for var forwarding
build/toolchain/gcc_toolchain.gni: further fixes for var forwarding

BUG=551343

Review-Url: https://codereview.chromium.org/1961433003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#392040}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk