kbuild: explicitly run mksysmap as sed script from link-vmlinux.sh

In commit b18b047 ("kbuild: change scripts/mksysmap into sed
script"), the mksysmap script was transformed into a sed script,
made directly executable with "#!/bin/sed -f". Apparently, the path to
sed is different on NixOS.

The shebang can't use the env command, otherwise the "sed -f" command
would be treated as a single argument. This can be solved with the -S
flag, but that is a GNU extension. Explicitly use sed instead of relying
on the executable shebang to fix NixOS builds without breaking build
environments using Busybox.

Fixes: b18b047 ("kbuild: change scripts/mksysmap into sed script")
Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Richard Acayan <mailingradian@gmail.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: flamingradian

scripts/link-vmlinux.sh

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