riscv disassembler leak

Commit 3f61a38b5e81 moved the disassembler subset_list from a static
variable to disassembler private_data.  It is now malloc'd in
riscv_init_disasm_info so should be freed when disassemble_free_target
runs.

	* riscv-dis.c (disassemble_free_riscv): Free subset_list.