kbuild: change scripts/mksysmap into sed script

The previous commit removed the subshell execution from scripts/mksysmap,
which is now simple enough to become a sed script.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/link-vmlinux.sh

@@ -193,7 +193,7 @@ kallsyms_step()
 mksysmap()
 {
 	info NM ${2}
-	${CONFIG_SHELL} "${srctree}/scripts/mksysmap" ${1} ${2}
+	${NM} -n "${1}" | "${srctree}/scripts/mksysmap" > "${2}"
 }
 
 sorttable()