clang: Add packageconfig knob for opt-viewer

clang: Add packageconfig knob for opt-viewer

The optimization viewer is a web-based tool that helps developers visualize
and analyze LLVM's optimization passes. It generates reports showing
Which optimizations were applied to specific functions.
Why certain optimizations were or weren't performed.
Performance remarks and missed optimization opportunities
Interactive views of the optimization pipeline.

Its a nice add-on, keep it disabled by default.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>