As with any PC hardware which implements ACPI (basically every modern system), you use the "power button override." See ACPI v. 3.0b section 4.7.2.3 "Sleeping/Wake Control. The description of the PM1.PWRBTN_STS bit in Table 4-11 gives further details.

In short: press and hold the power button for 4 seconds, and the system will transition straight to the G2/S5 Soft Off state.