kbuild, LLVMLinux: Add -Werror to cc-option to support clang

kbuild, LLVMLinux: Add -Werror to cc-option to support clang

Clang will warn about unknown warnings but will not return false
unless -Werror is set. GCC will return false if an unknown
warning is passed.

Adding -Werror make both compiler behave the same.

[arnd: it turns out we need the same patch for testing whether -ffunction-sections
       works right with gcc. I've build tested extensively with this patch
       applied, so let's just merge this one now.]

Signed-off-by: Mark Charlebois <[email protected]>
Signed-off-by: Behan Webster <[email protected]>
Reviewed-by: Jan-Simon Möller <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>
Acked-by: Kees Cook <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>