The gold linker support and gold plugin variants now use the same name.
Trying to apply use-flag-style discipline here despite the fact gold has
other implications for clang, this way globally enabling gold will have
a more consistent effect if that becomes possible. The gold support in
gcc could use more testing to ensure it works consistently, but as long
as a binutils including gold is used the gcc configure tends to pick it
up, and it seems to work with 5.3.0 at least.
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