gdb: restrict configure error with all targets and 64 bit bfd to mips

The recent commit b601c58034ed755fb765fc13782b6876bffd25d4 causes the
gdb configure to fail if --enable-targets=all was requested, but 64 bit
bfd was not enabled. This was due to a build failure first reported
against mips, and that I also encountered building on a 32 bit mips
system, but that looked like a general failure.

Further examination showed that this is, in fact, mips-specific (or at
least, not completely generic) as other targets like debian-i386 and
32-bit arm could build all targets just fine.

This commit restricts the new error to only trigger in mips hosts.

Approved-By: Andrew Burgess <aburgess@redhat.com>