scripts: make sure scripts are invoked via $(PYTHON)

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>

configure

@@ -1767,7 +1767,7 @@ if test -n "$gdb_bin"; then
     gdb_version=$($gdb_bin --version | head -n 1)
     if version_ge ${gdb_version##* } 9.1; then
         echo "HAVE_GDB_BIN=$gdb_bin" >> $config_host_mak
-        gdb_arches=$("$source_path/scripts/probe-gdb-support.py" $gdb_bin)
+        gdb_arches=$($python "$source_path/scripts/probe-gdb-support.py" $gdb_bin)
     else
         gdb_bin=""
     fi