syscall: accept permission errors in TestExecPtrace
syscall: accept permission errors in TestExecPtrace

Fixes the test on the linux-arm64-packet builder.

Change-Id: Icaf9edf9542f4f6e3791ca43298a1e7051eaa576
Reviewed-on: https://go-review.googlesource.com/c/go/+/182378
Run-TryBot: Elias Naur <1d6e1cf70ec6f9ab28d3ea4b27a49a77654d370e@eliasnaur.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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