gdb/mips: rewrite show_mask_address

This commit is similar to the previous commit, but in this case GDB is
actually relying on undefined behaviour.

Consider building GDB for all targets on x86-64/GNU-Linux, then doing
this:

  (gdb) show mips mask-address
  Zeroing of upper 32 bits of 64-bit addresses is auto.
  The 32 bit address mask is set automatically.  Currently disabled
  (gdb)

The 'show mips mask-address' command ends up in show_mask_address in
mips-tdep.c, and this function does this:

  mips_gdbarch_tdep *tdep
    = (mips_gdbarch_tdep *) gdbarch_tdep (target_gdbarch ());

Later we might pass TDEP to mips_mask_address_p.  However, in my
example above, on an x86-64 native target, the current target
architecture will be an x86-64 gdbarch, and the tdep field within the
gdbarch will be of type i386_gdbarch_tdep, not of type
mips_gdbarch_tdep, as a result the cast above was incorrect, and TDEP
is not pointing at what it thinks it is.

I also think the current output is a little confusing, we appear to
have two lines that show the same information, but using different
words.

The first line comes from calling deprecated_show_value_hack, while
the second line is printed directly from show_mask_address.  However,
both of these lines are printing the same mask_address_var value.  I
don't think the two lines actually adds any value here.

Finally, none of the text in this function is passed through the
internationalisation mechanism.

It would be nice to remove another use of deprecated_show_value_hack
if possible, so this commit does a complete rewrite of
show_mask_address.

After this commit the output of the above example command, still on my
x86-64 native target is:

    (gdb) show mips mask-address
    Zeroing of upper 32 bits of 64-bit addresses is "auto" (current architecture is not MIPS).

The 'current architecture is not MIPS' text is only displayed when the
current architecture is not MIPS.  If the architecture is mips then we
get the more commonly seen 'currently "on"' or 'currently "off"', like
this:

  (gdb) set architecture mips
  The target architecture is set to "mips".
  (gdb) show mips mask-address
  Zeroing of upper 32 bits of 64-bit addresses is "auto" (currently "off").
  (gdb)

All the text is passed through the internationalisation mechanism, and
we only call gdbarch_tdep when we know the gdbarch architecture is
bfd_arch_mips.

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