kbuild: add mod(name,file)_flags to assembler flags for module objects

In order to create the file at build time, modules.builtin.ranges, that
contains the range of addresses for all built-in modules, there needs to
be a way to identify what code is compiled into modules.

To identify what code is compiled into modules during a kernel build,
one can look for the presence of the -DKBUILD_MODFILE and -DKBUILD_MODNAME
options in the compile command lines.  A simple grep in .*.cmd files for
those options is sufficient for this.

Unfortunately, these options are only passed when compiling C source files.
Various modules also include objects built from assembler source, and these
options are not passed in that case.

Adding $(modfile_flags) to modkern_aflags (similar to modkern_cflags), and
adding $(modname_flags) to a_flags (similar to c_flags) makes it possible
to identify which objects are compiled into modules for both C and
assembler source files.  While KBUILD_MODFILE is sufficient to generate
the modules ranges data, KBUILD_MODNAME is passed as well for consistency
with the C source code case.

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Tested-by: Sam James <sam@gentoo.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>