gdb/riscv: Give user-friendly names for CSRs

gdb/riscv: Give user-friendly names for CSRs

The recent commit:

    commit 0dbfcfffe9abbc5198bce95eb8c66b6bc9b364be
    Date:   Tue Oct 16 22:40:09 2018 +0100

        gdb/riscv: Fix register access for register aliases

broke the CSR names for RISC-V, now all of the CSRs have names like,
csr0, csr1, csr2, etc.  This commit restores the previous
user-friendly names.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_register_name): Use the user-friendly names
	for CSRs.