Cyclone V clock pin driver output impedance

There are several IO standards that can drive single ended 50 ohm or differential 100 ohm lines.

The preferred termination scheme to achieve full 2.5, 3.0 or 3.3V CMOS level would be a 50 ohm source side termination and high impedance load side termination. You can also feed a 50 ohm load with the 50 ohm transmission line, resulting in halved receiver level. What's your intended load impedance?

For the 50 ohm source side termination, you'll usually place a series resistor that adds with the driver output impedance to 50 ohms. Expect a driver output impedance around 15 ohms for high drive strength. Detail information (pin I/V characteristics) for different IO standards can be found in Ibis files.

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Most pins could only provide something like 8 to 12 mA.

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Don't confuse the CMOS/TTL drive strength specification with maximal output current. You should however keep the maximum ratings in device datasheet.