Use common_val_print in riscv-tdep.c

Use common_val_print in riscv-tdep.c

This changes some spots in riscv-tdep.c to use common_val_print rather
than val_print.

gdb/ChangeLog
2020-03-13  Tom Tromey  <tom@tromey.com>

	* riscv-tdep.c (riscv_print_one_register_info): Use
	common_val_print.