[fuzz] Add default path for Mesa Vulkan driver

Adds support in the fuzzers to look for the Mesa driver in a
pre-defined location, the same directory as the binary. This mirrors
how dxc is found, with there being a CLI flag to override this value
as needed.

This is done so that ClusterFuzz targets don't require command line
flags to run with the Mesa driver, since flags to the binary are not
supported. This retains the ability when running locally to supply a
flag to override the value.

The build rules for Mesa are also changed to add a post build script
that will copy the results to the route of the build directory, and
rewrite the config JSON file.

Since this behaviour is desired for both the tint_wgsl_fuzzer and
tint_ir_fuzzer, a new common code location target is added for the
fuzzers. Existing duplicated code is not migrated to use this location
yet. Future CLs will do that.

Various bits of documentation are updated to reference this new output
layout.

Bug: 506912899
Change-Id: I0f11f407dd12781b41ab6b1b84893c3c5a95f7f7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/305916
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>