MFC r316029: lld: hack version and help output for compatibility with libtool

GNU libtool checks the output from invoking the linker with --version
and --help, in order to determine the linker "flavour" and the command-
ine arguments to use for various link operations (e.g. generating shared
libraries). To detect GNU ld it looks for the strings "GNU" and
"supported targets:.*elf". Since LLD is compatible with GNU ld we
include those same strings to fool libtool.

Quoting from a comment in the change:

This is somewhat ugly hack, but in reality, we had no choice other
than doing this. Considering the very long release cycle of Libtool,
it is not easy to improve it to recognize LLD as a GNU compatible
linker in a timely manner. Even if we can make it, there are still a
lot of "configure" scripts out there that are generated by old
version of Libtool. We cannot convince every software developer to
migrate to the latest version and re-generate scripts. So we have
this hack.

Upstream LLVM revisions r298532, r298568, r298591

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