[gn build] make stage2_unix_toolchain set clang_base_path

[gn build] make stage2_unix_toolchain set clang_base_path

This fixes the build of compiler-rt on macOS when _not_ using
clang_base_path in args.gn: Xcode clang knows where to find the
SDK, but regular clang doesn't and needs a -isysroot parameter.
We correctly add that parameter when clang_base_path is set,
but else we omit it. If clang_base_path was not set, we also
didn't add the flag for stage2_unix_toolchain() when we build
compiler-rt with just-built clang.

Make stage2_unix_toolchain() use clang_base_path instead of setting
cc / cxx. It's less code, and it gets things like this right.