UPSTREAM: kbuild: drop -Wno-unknown-warning-option from clang options

Since commit c3f0d0bc ("kbuild, LLVMLinux: Add -Werror to
cc-option to support clang"), cc-option and friends work nicely
for clang.

However, -Wno-unknown-warning-option makes clang happy with any
unknown warning options even if -Werror is specified.

Once -Wno-unknown-warning-option is added, any succeeding call of
cc-disable-warning is evaluated positive, then unknown warning
options are accepted.  This should be dropped.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
(cherry picked from commit a0ae981e)
Signed-off-by: Greg Hackmann <ghackmann@google.com>

Change-Id: I0535e20fbcecc2d431e9f08b1f274c5d96626af1