runtime: crash the GC when clobberdead pointer is seen

When -clobberdead compiler flag is set, the compiler inserts
instructions that set dead slots a specific value. If the GC sees
this value as a live pointer, something is probably wrong. Crash.

Only do this on AMD64 for now, as it is the only platform where
compiler's clobberdead mode is implemented. And on AMD64 the
clobberdead address can never be a valid address.

Change-Id: Ica687b132b5d3ba2a062500d13264fa730405d11
Reviewed-on: https://go-review.googlesource.com/c/go/+/310330


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>