clang: add option for llvm-exegesis

clang: add option for llvm-exegesis

Quoting from the README:

    llvm-exegesis is a benchmarking tool that accepts or generates
    snippets and can measure characteristics of those snippets by
    executing it while keeping track of performance counters.

Potentially useful but not essential. This reduces the size of llvm-bin
by ~30MB and llvm-dbg by ~1100MB.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>