[gn build] Use llvm-ar when clang_base_path is specified

Only applies linux for now.

This prevents warnings with use_thinlto like
  bfd plugin: LLVM gold plugin has failed to create LTO module: Not an int attribute (Producer: 'LLVM15.0.0git' Reader: 'LLVM 13.0.1')

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D125399