-mabi= was incorrectly claimed before D134671. -mabi=sysv appears to be
somewhat common in open-source packages, even if it was not intended to
be supported by Clang.
(For common options supported by multiple architectures, it's easy to
forget to report an error on unsupported targets. Unfortunately
the driver infrastructure doesn't make this less error-prone.)
On x86, support -mabi=sysv for non-Windows targets and -mabi=ms for Windows,
and remove the spurious -Wunused-command-line-argument warning.
With this change, all popular architectures claim -mabi=, so we don't
have to worry much about -Wunused-command-line-argument for other
architectures.