clang: Extract default lnosys flag into a config

For cases where we rely on symbols from custom libraries instead of
libnosys, adding libnosys by default fails with duplicate symbol errors.
Extracting `lnosys` ldflag into a config allows the downstream projects
to remove it gracefully if required.

Bug: b/244452330
Change-Id: I4243f30a033380865d3159438955020ce41b3dcd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/133110
Reviewed-by: Petr Hosek <phosek@google.com>
Commit-Queue: Prabhu Karthikeyan Rajasekaran <prabhukr@google.com>
Reviewed-by: Keir Mierle <keir@google.com>