kbuild: uapi: use -fsyntax-only rather than -S

The UAPI header tests are checking that the generated headers do not
have syntax errors. There's no need to run the rest of the compilation
pipeline after semantic analysis has run. Replace -S -o /dev/null with
-fsyntax-only.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>