kbuild: do not try to parse *.cmd files for objects provided by compiler

Guenter Roeck reported the build breakage for parisc and csky.
I confirmed nios2 and openrisc are broken as well.

The reason is that they borrow libgcc.a from the toolchains.

For example, see this line in arch/parisc/Makefile:

    LIBGCC          := $(shell $(CC) -print-libgcc-file-name)

Some objects in libgcc.a are linked to vmlinux.o, but they do not have
.*.cmd files.

Obviously, there is no EXPORT_SYMBOL in external objects. Ignore them.

(Most of the architectures import library code into the kernel tree.
Perhaps those 4 architectures can do similar, but I do not know how
challenging it is.)

Fixes: f292d875

 ("modpost: extract symbol versions from *.cmd files")
Link: https://lore.kernel.org/linux-kbuild/20220528224745.GA2501857@roeck-us.net/T/#mac65c20c71c3e272db0350ecfba53fcd8905b0a0
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>