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.
0ff80bf7b9