opcodes/arm: don't pass non-string literal to printf like function

opcodes/arm: don't pass non-string literal to printf like function

The earlier commit:

  commit 6576bffe6cbbb53c5756b2fccd2593ba69b74cdf
  Date:   Thu Jul 7 13:43:45 2022 +0100

      opcodes/arm: add disassembler styling for arm

introduced two places where a register name was passed as the format
string to the disassembler's fprintf_styled_func callback.  This will
cause a warning from some compilers, like this:

  ../../binutils-gdb/opcodes/arm-dis.c: In function ‘print_mve_vld_str_addr’:
  ../../binutils-gdb/opcodes/arm-dis.c:6005:3: error: format not a string literal and no format arguments [-Werror=format-security]
   6005 |   func (stream, dis_style_register, arm_regnames[gpr]);
        |   ^~~~

This commit fixes these by using "%s" as the format string.