riscv: retire the FPE kernel option

[mhorne]

Summary

We always build the kernel floating point support. Now that the
riscv64sf userspace variant has been removed the option is required for
correct operation.

Alternatively it could be moved to DEFAULTS, if there is still some
conceivable reason to keep this.

[jrtc27]

I agree FPE can go, just a minor comment

sys/riscv/riscv/exec_machdep.c

283

Probably best to leave this alone and keep the patch purely about the ifdefs (and moving headers inline where style violations are otherwise introduced)?

[jhb]

Kill it with fire.

+1 on Jess's suggestion about leaving the one line of whitespace.