Makefile: use -Wno-main in the full kernel tree

When using gcc (SUSE Linux) 7.5.0 (on openSUSE 15.3), I see a build
warning:

  kernel/trace/trace_osnoise.c: In function 'start_kthread':
  kernel/trace/trace_osnoise.c:1461:8: warning: 'main' is usually a function [-Wmain]
    void *main = osnoise_main;
          ^~~~

Quieten that warning by using "-Wno-main".  It's OK to use "main" as a
declaration name in the kernel.

Build-tested on most ARCHes.

[ v2: only do it for gcc, since clang doesn't have that particular warning ]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/lkml/20210813224131.25803-1-rdunlap@infradead.org/
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>