checkasm: Don't use readtime for checkasm seed
On ARM, the readtime implementations are instructions that might not always be allowed at runtime (depending on whether the kernel has allowed user mode code to access the cycle counter registers). In order to allow building checkasm with the option for benchmarking, while still running on devices where benchmarking isn't possible, don't use readtime anywhere unless --bench has been specified. Use GetTickCount for the seed on windows, and gettimeofday on unix.
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