RISC-V: Add testcase for DWARF register numbers

Although it had csr-dw-regnums.d (for CSRs), it didn't have DWARF register
number test for GPRs/FPRs.

This commit adds dw-regnums.{s,d} to test such registers.

gas/ChangeLog:

	* testsuite/gas/riscv/dw-regnums.s: New DWARF register number test
	for GPRs/FPRs.
	* testsuite/gas/riscv/dw-regnums.d: Likewise.