RISC-V: Fix wrong use of s0 register name.

RISC-V: Fix wrong use of s0 register name.

s0 is listed as both an int register name and an FP register name.  The FP reg
name is wrong.  This looks like a simple editting error, and has an easy fix.
Tested with riscv64-linux build and check, with no regressions.

	gdb/
	* riscv-tdep.c (riscv_freg_feature): Drop s0 name from f8.