kasan: simplify and clarify Makefile

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.

Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>
Acked-by: Marco Elver <elver@google.com>
Link: https://lore.kernel.org/r/20240814161052.10374-1-andrey.konovalov@linux.dev
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>