kbuild: fix empty ${PYTHON} in scripts/link-vmlinux.sh

The two commits

d8d2d38275c1 ("kbuild: remove PYTHON variable")
a8cccdd95473 ("init: lto: ensure initcall ordering")

were applied in the same development cycle, into two different trees.

After they were merged together, this ${PYTHON} expands to an empty
string.

Therefore, ${srctree}/scripts/jobserver-exec is executed directly.
(it has the executable bit set)

This is working but let's fix the code into the intended form.

Signed-off-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Nick Desaulniers <[e-mail]>
Reviewed-by: Sedat Dilek <[e-mail]>

scripts/link-vmlinux.sh

···
 {
 	info GEN .tmp_initcalls.lds
 
-	${PYTHON} ${srctree}/scripts/jobserver-exec		\
+	${PYTHON3} ${srctree}/scripts/jobserver-exec		\
 	${PERL} ${srctree}/scripts/generate_initcall_order.pl	\
 		${KBUILD_VMLINUX_OBJS} ${KBUILD_VMLINUX_LIBS}	\
 		> .tmp_initcalls.lds