gdb: rename target_fileio_stat to target_fileio_lstat

gdb: rename target_fileio_stat to target_fileio_lstat

In the following commits I added the target_fileio_stat function, and
the target_ops::fileio_stat member function:

  * 08a115cc1c4 gdb: add target_fileio_stat, but no implementations yet
  * 3055e3d2f13 gdb: add GDB side target_ops::fileio_stat implementation
  * 6d45af96ea5 gdbserver: add gdbserver support for vFile::stat packet
  * 22836ca8859 gdb: check for multiple matching build-id files

Unfortunately, I messed up when adding this API.  The actual
underlying call is lstat, not stat.

This commit tries to clear up some of the confusion by renaming things
to target_fileio_lstat and target_ops::fileio_lstat.

After this change the function names now match the underlying
implementation.

One problem remains though.  In order to support target_fileio_stat
for remote target the above patches added the vFile:stat packet to GDB
and gdbserver.  The implementation of this packet still does an lstat
though, which is a bit of a shame.  I'm going to try and fix that in
later commits.

This commit is just a rename within GDB, there should be no user
visible changes.

Approved-By: Tom Tromey <tom@tromey.com>