i86pc fpu_subr.c isn't really platform-specific
Right now we have a single copy of the
fpu_subr.c file that is in an i86pc-specific directory, though also shared with i86xpv. It also doesn't really have anything in it that is specific to the platform (other than xpv ifdefs) and the rest of it is actually just architectural content. We should just move this into uts/intel/os as that is where it appears to actually belong. Merging it with the rest of the FPU code in uts/intel is left for another time, though a small bit has moved around with #15254.