syscall: skip an unsupported test case on android

Lookup is not supported on android, and the test
syscall/exec_linux_test.go which relies on it will fail on
android/arm64.

Fixes #27327

Change-Id: I6fdb8992d4634ac7e3689360ff114e9431b5e90c
Reviewed-on: https://go-review.googlesource.com/131995


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>