mm, kasan: Update kasan docs to indicate arm64 support

KASAN has been supported on arm64 since 39d114ddc682 ("arm64: add KASAN
support"). Update the docs to indicate this.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kasan.txt

@@ -12,7 +12,7 @@ KASAN uses compile-time instrumentation for checking every memory access,
 therefore you will need a GCC version 4.9.2 or later. GCC 5.0 or later is
 required for detection of out-of-bounds accesses to stack or global variables.
 
-Currently KASAN is supported only for x86_64 architecture.
+Currently KASAN is supported only for x86_64 and arm64 architecture.
 
 1. Usage
 ========