KVM: selftests: Probe -no-pie with actual CFLAGS used to compile

Probe -no-pie with the actual set of CFLAGS used to compile the tests,
clang whines about -no-pie being unused if the tests are compiled with
-static.

  clang: warning: argument unused during compilation: '-no-pie'
  [-Wunused-command-line-argument]

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221213001653.3852042-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>