Respect objdump -m flag

This resolves at least part of #19; it might also fix gdb disassembly.

opcodes/riscv-dis.c

@@ -406,8 +406,12 @@ riscv_disassemble_insn (bfd_vma memaddr, insn_t word, disassemble_info *info)
     {
       int xlen = 0;
 
-      /* The incoming section might not always be complete.  */
-      if (info->section != NULL)
+      /* If XLEN is not known, get its value from the ELF class.  */
+      if (info->mach == bfd_mach_riscv64)
+	xlen = 64;
+      else if (info->mach == bfd_mach_riscv32)
+	xlen = 32;
+      else if (info->section != NULL)
 	{
 	  Elf_Internal_Ehdr *ehdr = elf_elfheader (info->section->owner);
 	  xlen = ehdr->e_ident[EI_CLASS] == ELFCLASS64 ? 64 : 32;