runtime/pprof: TestCPUProfileLabel is flaky on aix-ppc64

[bcmills]

2020-04-08T18:37:38-d5e1b7c/aix-ppc64
2020-03-17T01:24:51-7ec4adb/aix-ppc64
2019-12-20T20:12:18-b234fdb/aix-ppc64
2019-11-19T19:21:57-e0306c1/aix-ppc64
2019-09-26T17:37:02-a37f2b4/aix-ppc64

Not clear to me whether this is a builder issue, an AIX platform issue, or a timing-sensitive test.

CC @hyangah for pprof, @trex58 @Helflym for aix, @andybons @toothrot @cagedmantis @dmitshur for builders.

[Helflym]

I think that the same than #36084. The test seems to be stuck in a usleep called by svmon as shown by the samples taken.

184: 0x10002f304 (runtime.usleep:509) 0x10003fe33 (runtime.sysmon:4333) 0x1000373e3 (runtime.mstart1:1238) 0x1000372cb (runtime.mstart:1203)

It's not just the builder as I was able to get the same failure on my local machine.
Is it possible that a deadlock occurs between sysmon and pprof functions ?