Some scripts are invoked via the first "python3" binary in the PATH,
because they are executable and their shebang line is "#! /usr/bin/env
python3".  To enforce usage of $(PYTHON), make them nonexecutable.
Scripts invoked via meson need nothing else, and meson-buildoptions.py
is already using $(PYTHON).  For probe-gdb-support.py however the
invocation in the configure script has to be adjusted.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>