gdb/riscv: Make some target description functions constant

gdb/riscv: Make some target description functions constant

Makes more of the interface related to fetching target descriptions
constant.

gdb/ChangeLog:

	* arch/riscv.h (riscv_create_target_description): Make return type
	const.
	* arch/riscv.c (riscv_create_target_description): Likewise.
	* riscv-tdep.c (riscv_find_default_target_description): Likewise.