runtime, cmd/link/internal/ld: disable memory profiling when data unreachable

If runtime.MemProfile is unreachable, default to not collecting any
memory profiling samples, to save memory on the hash table.

Fixes #42347

Change-Id: I9a4894a5fc77035fe59b1842e1ec77a1182e70c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/299671


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Keith Randall <khr@golang.org>