Default Linux toolchain linker to gold
The BFD linker on most of the Linux box is unable to link Swift objects.
Currently the Swift driver hard-codes forcing the default linker to
gold, but we want to support configuring this, so we need build-script
to force it at the clang level instead.
Cherry-Picks:
d63152fd8070
7596d0091600
826255254e8f
