arm64: Don't use uxth for extending a register

armasm64 fails to assemble this: error A2173: syntax error in expression sub x7, x7, w4, uxth

This clearly is a bug in armasm64, and will be reported. For now, this workaround should be harmless though, as we've just loaded the register with ldrh, so the upper parts of the register should be zeroed.