gn: bootstrap: always use cxx for linking

LD may be set to something like "ld.bfd" or "ld.gold". We want to let
the compiler invoke the linker instead of calling these directly.

Bug: https://bugs.gentoo.org/597218
Review-Url: https://codereview.chromium.org/2422203002
Cr-Original-Commit-Position: refs/heads/master@{#425723}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 11aa518d0b8d280fdc5958ae440a4f799fa6ac56