gen_compile_commands: add assembly files to compilation database

Like C source files, tooling can find it useful to have the assembly
source file compilation recorded.

The .S extension appears to used across all architectures.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Fangrui Song <maskray@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>