kasan: simplify and clarify Makefile

When KASAN support was being added to the Linux kernel, GCC did not yet
support all of the KASAN-related compiler options.  Thus, the KASAN
Makefile had to probe the compiler for supported options.

Nowadays, the Linux kernel GCC version requirement is 5.1+, and thus we
don't need the probing of the -fasan-shadow-offset parameter: it exists in
all 5.1+ GCCs.

Simplify the KASAN Makefile to drop CFLAGS_KASAN_MINIMAL.

Also add a few more comments and unify the indentation.

[andreyknvl@gmail.com: comments fixes per Miguel]
  Link: https://lkml.kernel.org/r/20240814161052.10374-1-andrey.konovalov@linux.dev
Link: https://lkml.kernel.org/r/20240813224027.84503-1-andrey.konovalov@linux.dev


Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>