[Driver] Ignore unimplemented -mtune= for ARM/PowerPC

This compensates for 8f0c901c which enabled
-Wunused-command-line-argument for unimplemented -mtune= in the generic code.
Ignoring -mtune= appears to be longstanding and the error-free behavior in the
presence of -Werror is unfortunately relied on by the Linux kernel's arm and
powerpc ports. Ignore the warnings for the upcoming 15.0.0 branch and will
implement functionality to fill the test gap soon.

Link: https://github.com/ClangBuiltLinux/linux/issues/1674