Handle unaligned futex addresses

[mbaxter]

Description

Following up on #11605, we should add additional tests that validate FPVM behavior when a futex operation has an unaligned address argument (argument a0 here).

Currently, the VM will panic when we try to access unaligned memory, but we should consider either silently aligning the futex address argument as we do for memory reads and writes, or else explicitly panicking if this address is not aligned.