RISC-V: Make mapping symbol checking consistent

There were two places where the mapping symbols are checked but had
different conditions.

-    riscv_get_map_state:          "$x" or "$d"
-    riscv_elf_is_mapping_symbols: Starts with either "$x" or "$d"

Considering recent proposals and Nelson's "RISC-V: Output mapping
symbols with ISA string once .option arch is used.", it's better to make
symbol checking consistent (whether the symbol _starts_ with "$[xd]").

opcodes/ChangeLog:

	* riscv-dis.c (riscv_get_map_state): Change the condition for
	consistency.

opcodes/riscv-dis.c

@@ -880,9 +880,9 @@ riscv_get_map_state (int n,
     return false;
 
   name = bfd_asymbol_name(info->symtab[n]);
-  if (strcmp (name, "$x") == 0)
+  if (startswith (name, "$x"))
     *state = MAP_INSN;
-  else if (strcmp (name, "$d") == 0)
+  else if (startswith (name, "$d"))
     *state = MAP_DATA;
   else
     return false;