On be3776ec2a, we changed outdir to
absolute path.
This causes "unknown target" error when we build Scylla using the relative
path something like "ninja build/dev/scylla", since the target name
become absolte path.
Revert the change to able to build with the relative path.
Also, change optimized_clang.sh to use relative path for --builddir,
since we reference "../../$builddir/SCYLLA-*-FILE" when we build
submodule, it won't work with absolute path.
Fixes#18321Closesscylladb/scylladb#18338
This is a different way attempting to combine building an optimized clang (using LTO, PGO and BOLT, based on compiling ScyllaDB) to dbuild. Per Avi's request, there are 3 options: skip this phase (which is the current default), build it and build + install it to the default path.
Fixes: #10985Fixes: scylladb/scylla-enterprise#2539
