For those who are curious, it's the highest bit in port 70h (this port is also used as an index register when accessing the CMOS/RTC): https://wiki.osdev.org/CMOS#Non-Maskable_Interrupts
Thre's also another set of NMI control bits in port 61h on an AT. It's worth noting that the port 70h and 61h controls are still there in the 2022 Intel 600-series chipsets, almost 40 years later; look at pages "959" and "960" of this: https://cdrdv2.intel.com/v1/dl/getContent/710279?fileName=71...