CONFIG_ARC_DSP_USERSPACE is not available for thedefaultarchitecture x86.
Result is shown for architecture arc

Support DSP for userspace apps

configname: CONFIG_ARC_DSP_USERSPACE

DSP support
└─>Support DSP for userspace apps
In linux kernel since version 5.1 (release Date: 2019-05-05)  
DSP extension presence in HW, support save / restore DSP registers to
run DSP-enabled userspace applications