bfd: add new bfd_cache_size() function

bfd: add new bfd_cache_size() function

In GDB we have a problem with the BFD cache.

As GDB runs for a potentially extended period of time, if the BFD
cache holds a file descriptor for an open on-disk file, this can, on
some targets (e.g. Win32) prevent the OS writing to the file.

This might, for example, prevent a user from recompiling their
executable as GDB is (via the BFD cache) holding an open reference to
that file.

Another problem, relates to bfd_stat, for BFDs that are using the BFD
cache (i.e. they call cache_bstat to implement bfd_stat).  The
cache_bstat function finds the BFD in the cache, opening the file if
needed, and then uses fstat on the open file descriptor.

What this means is that, if the on-disk file changes, but the cache
was holding an open reference to the file, the bfd_stat will return
the 'struct stat' for the old file, not the new file.

Now, for this second problem, we might be tempted to make use of an
actual stat call, instead of calling bfd_stat, however, this isn't
ideal as we have some BFDs that use a custom iovec, and implement the
various functions over GDB's remote protocol.  By using bfd_stat we
can have a single call that should work for both local files, and for
remote files.

To solve both of these problems GDB has calls to bfd_cache_close_all
sprinkled around its code base.  And in theory this should work fine.

However, I recently ran into a case where we had missed a
bfd_cache_close_all call, and as a result some BFDs were held open.
This caused a bfd_stat call to return an unexpected result (old file
vs new file).

What I'd like is some way within GDB that I can do:

  gdb_assert ( /* Nothing is held open in the cache.  */ );

As this would allow GDB to quickly identify when we've missed some
bfd_cache_close_all calls.

And so, to support this, I would like to add a new bfd_cache_size
function.  This function returns an integer, which is the number of
open files in the cache.  I can then start adding:

  gdb_assert (bfd_cache_size() == 0);

to GDB in some strategic spots, and start fixing all of the missing
bfd_cache_close_all calls that crop up as a result.