kbuild: disallow multi-word in M= or KBUILD_EXTMOD

$(firstword ...) in scripts/Makefile.modpost was added by commit
3f3fd3c0 ("[PATCH] kbuild: allow multi-word $M in Makefile.modpost")
to build multiple external module directories.

It was a solution to resolve symbol dependencies when an external
module depends on another external module.

Commit 0d96fb20 ("kbuild: Add new Kbuild variable
KBUILD_EXTRA_SYMBOLS") introduced another solution by passing symbol
info via KBUILD_EXTRA_SYMBOLS, then broke the multi-word M= support.

  include $(if $(wildcard $(KBUILD_EXTMOD)/Kbuild), \
               $(KBUILD_EXTMOD)/Kbuild, $(KBUILD_EXTMOD)/Makefile)

... does not work if KBUILD_EXTMOD contains multiple words.

This feature has been broken for more than a decade. Remove the
bitrotten code, and stop parsing if M or KBUILD_EXTMOD contains
multiple words.

As Documentation/kbuild/modules.rst explains, if your module depends
on another one, there are two solutions:
  - add a common top-level Kbuild file
  - use KBUILD_EXTRA_SYMBOLS

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>