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.

gdb/arch/riscv.c

@@ -26,7 +26,7 @@
 
 /* See arch/riscv.h.  */
 
-target_desc *
+const target_desc *
 riscv_create_target_description (struct riscv_gdbarch_features features)
 {
   target_desc *tdesc = allocate_target_description ();