gdb: Improve formatting of 'show endian' messages

This commit changes the output of 'show endian'.  Here is a
session before this commit:

    (gdb) show endian
    The target endianness is set automatically (currently little endian)
    (gdb) set endian big
    The target is assumed to be big endian
    (gdb) show endian
    The target is assumed to be big endian
    (gdb)

After this commit the session now looks like this:

    (gdb) show endian
    The target endianness is set automatically (currently little endian).
    (gdb) set endian big
    The target is set to big endian.
    (gdb) show endian
    The target is set to big endian.
    (gdb)

The changes are:

  1. Each line ends with '.', and
  2. After setting the endianness GDB is now a little more assertive;
  'target is set to' not 'target is assumed to be', the user did just
  tell us after all!