gdb: only include mips and riscv targets if building with 64-bit bfd

gdb: only include mips and riscv targets if building with 64-bit bfd

While testing another patch I was trying to build different
configurations of GDB, and, during one test build I ran into a
problem, I configured with `--enable-targets=all
--host=i686-w64-mingw32` and saw this error while linking GDB:

  .../i686-w64-mingw32/bin/ld: mips-tdep.o: in function `mips_gdbarch_init':
  .../src/gdb/mips-tdep.c:8730: undefined reference to `disassembler_options_mips'
  .../i686-w64-mingw32/bin/ld: riscv-tdep.o: in function `riscv_gdbarch_init':
  .../src/gdb/riscv-tdep.c:3851: undefined reference to `disassembler_options_riscv'

So the `disassembler_options_mips` and `disassembler_options_riscv`
symbols are missing.

This turns out to be because mips-dis.c and riscv-dis.c, in which
these symbols are defined, are in the TARGET64_LIBOPCODES_CFILES list
in opcodes/Makefile.am, these files are only built when we are
building with a 64-bit bfd.

If we look further, into bfd/Makefile.am, we can see that all the
files matching elf*-riscv.lo are in the BFD64_BACKENDS list, as are
the elf*-mips.lo files, and (I know because I tried), the two
disassemblers do, not surprisingly, depend on features supplied from
libbfd.

So, though we can build most of GDB just fine for riscv and mips with
a 32-bit bfd, if I understand correctly, the final GDB
executable (assuming we could get it to link) would not understand
these architectures at the bfd level, nor would there be any
disassembler available.  This sounds like a pretty useless GDB to me.

So, in this commit, I move the riscv and mips targets into GDB's list
of targets that require a 64-bit bfd.  After this I can build GDB just
fine with the configure options given above.

This was discussed on the mailing list in a couple of threads:

  https://sourceware.org/pipermail/gdb-patches/2021-December/184365.html
  https://sourceware.org/pipermail/binutils/2021-November/118498.html

and it is agreed, that it is unfortunate that the 32-bit riscv and
32-bit mips targets require a 64-bit bfd.  If in the future this
situation ever changes then it would be expected that some (or all) of
this patch would be reverted.  Until then though, this patch allows
GDB to build when configured with --enable-targets=all, and when using
a 32-bit libbfd.