kbuild: replace LANG=C with LC_ALL=C

LANG gives a weak default to each LC_* in case it is not explicitly
defined. LC_ALL, if set, overrides all other LC_* variables.

  LANG  <  LC_CTYPE, LC_COLLATE, LC_MONETARY, LC_NUMERIC, ...  <  LC_ALL

This is why documentation such as [1] suggests to set LC_ALL in build
scripts to get the deterministic result.

LANG=C is not strong enough to override LC_* that may be set by end
users.

[1]: https://reproducible-builds.org/docs/locales/

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Reviewed-by: Matthias Maennich <maennich@google.com>
Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net> (mptcp)
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>