runtime: ignore EAGAIN from exec in TestCgoExecSignalMask
runtime: ignore EAGAIN from exec in TestCgoExecSignalMask

Fixes #27731

Change-Id: Ifb4d57923b1bba0210ec1f623d779d7b5f442812
Reviewed-on: https://go-review.googlesource.com/135995
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Michael Munday <19411bdeea90f848b8e02e9a1c63a7ce7cc699ea@ibm.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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