RISC-V: Test DWARF register number for "fp"

This commit adds "fp" (x8 or s0) to dw-regnums.{s,d}.

gas/ChangeLog:

	* testsuite/gas/riscv/dw-regnums.s: Add "fp".
	* testsuite/gas/riscv/dw-regnums.d: Likewise.