gdb: handle dis_style_sub_mnemonic disassembler style

In commit:

  commit 4f46c0bc36
  Date:   Mon Jul 4 17:45:25 2022 +0100

      opcodes: add new sub-mnemonic disassembler style

I added a new disassembler style dis_style_sub_mnemonic, but forgot to
add GDB support for this style.  Fix this oversight in this commit.

gdb/disasm.c

@@ -296,6 +296,7 @@ gdb_printing_disassembler::fprintf_styled_func (void *dis_info,
   switch (style)
     {
     case dis_style_mnemonic:
+    case dis_style_sub_mnemonic:
     case dis_style_assembler_directive:
       fputs_styled (txt, disasm_mnemonic_style.style (), stream);
       break;