linux.clone failing on some environments

The linux.clone test today verifies DR's handling of SYS_clone3. The checks the test performs are based on whether or not SYS_clone3 is defined: if it is not defined, we check for ENOSYS, but if it is, we check for the actual result. It isn't reliable to assume presence or absence of kernel syscall support based on whether the glibc version defines the corresponding SYS_ constant.