This intrinsic enables FIQ interrupts.
Typically, this intrinsic enables FIQ interrupts by clearing the F-bit in the CPSR. However, for v7-M, it clears the fault mask register (FAULTMASK). FIQ interrupts are not supported in v6-M.
The __enable_fiq intrinsic can only be executed in privileged modes, that is, in non-user modes. In User mode this intrinsic does not change the interrupt flags in the CPSR.