RISC-V: Cache the latest mapping symbol and its boundary.

RISC-V: Cache the latest mapping symbol and its boundary.

This issue was reported from https://github.com/riscv-collab/riscv-gnu-toolchain/issues/1188

Current flow:
1) Scan any mapping symbol less than this instruciton.
2) If not found, did a backward search.

The flow seems not big issue, let run an example here:

$x:
0x0 a   <--- Found at step 1
0x4 b   <--- Not found in step 1, but found at step 2
0x8 c   <--- Not found in step 1, but found at step 2
$d
0x12 .word 1234 <-- Found at step 1

The instruciton didn't have the same address with mapping symbol will
still did backward search again and again.

So the new flow is:
1) Use the last mapping symbol status if the address is still within the range
   of the current mapping symbol.
2) Scan any mapping symbol less than this instruciton.
3) If not found, did a backward search.
4) If a proper mapping symbol is found in either step 2 or 3, find its boundary,
   and cache that.

Use the same example to run the new flow again:

$x:
0x0 a   <--- Found at step 2, the boundary is 0x12
0x4 b   <--- Cache hit at step 1, within the boundary.
0x8 c   <--- Cache hit at step 1, within the boundary.
$d
0x12 .word 1234 <-- Found at step 2, the boundary is the end of section.

The disassemble time of the test cases has been reduced from ~20 minutes to ~4
seconds.

opcode/ChangeLog
	PR 30282
	* riscv-dis.c (last_map_symbol_boundary): New.
	(last_map_state): New.
	(last_map_section): New.
	(riscv_search_mapping_symbol): Cache the result of latest
	mapping symbol.