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-Commit-Position: refs/heads/master@{#425723}
1 file changed