KVM: selftests: Include lib.mk before consuming $(CC)

Include lib.mk before consuming $(CC) and document that lib.mk overwrites
$(CC) unless make was invoked with -e or $(CC) was specified after make
(which makes the environment override the Makefile).  Including lib.mk
after using it for probing, e.g. for -no-pie, can lead to weirdness.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221213001653.3852042-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>