perf python: Ignore unused command line arguments when building with clang

perf python: Ignore unused command line arguments when building with clang

Noticed after switching to python3 by default on some older fedora
releases:

  35    38.20 fedora:27                     : FAIL clang version 5.0.2 (tags/RELEASE_502/final)
    clang-5.0: error: argument unused during compilation: '-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1' [-Werror,-Wunused-command-line-argument]
    clang-5.0: error: argument unused during compilation: '-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1' [-Werror,-Wunused-command-line-argument]
    error: command 'clang' failed with exit status 1

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>