runtime: frequent TestCrashDumpsAllThreads failures on `linux/s390x`

#!watchflakes
post <- goarch == "s390x" && pkg == "runtime" && test == "TestCrashDumpsAllThreads"

Go version

ca804118b1ff4ea6cd0b83808e140671d7e10f6f

What operating system and processor architecture are you using (go env)?

linux-s390x-ibm-race at ca804118b1ff4ea6cd0b83808e140671d7e10f6f

:: Running /data/golang/workdir/go/src/make.bash with args ["/data/golang/workdir/go/src/make.bash" "-force"] and env ["LANG=en_US.UTF-8" "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin" "HOME=/root" "LOGNAME=root" "USER=root" "SHELL=/bin/sh" "INVOCATION_ID=5ad2b81f504d4d4493366f364d13f297" "JOURNAL_STREAM=9:358624446" "META_BUILDLET_BINARY_URL=https://storage.googleapis.com/go-builder-data/buildlet.linux-s390x" "GO_STAGE0_NET_DELAY=0s" "GO_STAGE0_DL_DELAY=100ms" "WORKDIR=/data/golang/workdir" "GOROOT_BOOTSTRAP=/data/golang/workdir/go1.4" "GO_BUILDER_NAME=linux-s390x-ibm-race" "GO_BUILDER_FLAKY_NET=1" "GO_TEST_TIMEOUT_SCALE=5" "GOBIN=" "TMPDIR=/data/golang/workdir/tmp" "GOCACHE=/data/golang/workdir/gocache" "GOPLSCACHE=/data/golang/workdir/goplscache" "PWD=/data/golang/workdir/go/src"] in dir /data/golang/workdir/go/src

What did you do?

https://build.golang.org/log/5e29be06bbb7a8a4923d83a58972aa2a10677a94

What did you expect to see?

All tests passing.

What did you see instead?

--- FAIL: TestCrashDumpsAllThreads (0.11s)
    crash_unix_test.go:145: found 2 instances of main.crashDumpsAllThreadsLoop; expected 4
…
FAIL
FAIL	runtime	46.511s

(attn @golang/s390x)