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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