RISC-V: Search for mapping symbols from the last one found

With previous behaviour, multiple mapping symbols within the same
function would result in all the mapping symbols being searched.
This could slow down disassembly dramatically.

Multiple mapping symbols within a function can be a result of encoding
instructions as data, like sometimes seen in random instruction
generators.

opcodes/ChangeLog:

	* riscv-dis.c (riscv_search_mapping_symbol): Use last mapping
	symbol if it exists.