Don't block in getrandom() detection

[josephlr]

On Linux, we detect if getrandom is present by calling getrandom with
an empty buffer and seeing if ENOSYS is returned. However, even with
an empty buffer, this call will block unless we explicitly pass a flag.

This can be seen in the source for getrandom:
https://elixir.bootlin.com/linux/v5.1.8/source/drivers/char/random.c#L2043

This change adds a boolean parameter to syscall_getrandom to control
the blocking behavior. We now don't block in initialization, but do
block when actually reading random data.

[dhardy]

I think in practice it doesn't make much difference since detection happens on first use. Looks good to me however.

[newpavlov]

I also think that blocking is not a big deal. If kernel does not have getrandom syscall we immediately get ENOSYS, and if call blocks, the next getrandom call will block either way. But I guess with this PR we will get a slightly better behavior in multi-threaded programs on a system launch, i.e. instead of blocking on user-land synchronization, threads will be blocked on syscall.