runtime/pprof: mark TestCPUProfileMultithreadMagnitude as flaky

The Linux kernel starting in 5.9 and fixed in 5.16 has a bug that can
break CPU timer signal delivery on new new threads if the timer
interrupt fires during handling of the clone system call.

Broken CPU timer signal deliver will skew CPU profile results and cause
this test to fail.

There is currently no known workaround, so mark the test as flaky on
builders with known broken kernels.

For #49065

Change-Id: I37ceb9ea244869b0aab5cd9a36b27ca2f7e5d315
Reviewed-on: https://go-review.googlesource.com/c/go/+/363214


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>