misc/cgo/test: log error value in testSigprocmask
misc/cgo/test: log error value in testSigprocmask

The test has been flaky, probably due to EAGAIN, but let's find out
for sure.

Updates #25078

Change-Id: I5a5b14bfc52cb43f25f07ca7d207b61ae9d4f944
Reviewed-on: https://go-review.googlesource.com/109359
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,josharian/go,cloudflare/go,josharian/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,akutz/go,golang/go,nawawi/go,akutz/go,josharian/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,josharian/go,golang/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,josharian/go,josharian/go,CAFxX/go,CAFxX/go,akutz/go,CAFxX/go,golang/go,josharian/go,cloudflare/go,josharian/go,golang/go,golang/go,akutz/go,nawawi/go,akutz/go