[gdb/testsuite] Reduce errors after gdb exit in default_gdb_start

[gdb/testsuite] Reduce errors after gdb exit in default_gdb_start

When running test-case gdb.base/readnever.exp with target board readnow, and
without commit 96038148d0e "[gdb/testsuite] Skip gdb.base/readnever.exp with
target board readnow", we run into a bunch of errors, starting with:
...
spawn gdb -nw -nx -data-directory data-directory -ex set sysroot -readnow \
  --readnever^M
gdb: '--readnow' and '--readnever' cannot be specified simultaneously^M
ERROR: : spawn id exp9 not open
    while executing
"expect {
-i exp9 -timeout 10
        -re "$gdb_prompt $" {
            verbose "Setting height to 0." 2
        }
...

The illegal combination of --readnow and --readnever causes gdb to start,
print an error message and exit.  There's a gdb_expect in default_gdb_start
that is supposed to detect the initial gdb prompt and handle related problems,
but since there's no eof case it succeeds, and default_gdb_start continues as
if the gdb prompt had been detected, causing the error above.

Fix this by adding an eof case to the gdb_expect, such that we have the more
accurate:
...
ERROR: (eof) GDB never initialized.
...

Further errors are triggered in clean_restart, because we're not testing for
gdb_start success.  Fix this by detecting gdb_start failure, and bailing out.

Finally, we're running into further errors in gdb.base/readnever.exp because
we're not testing for clean_restart success.  Fix this by making clean_restart
return -1 upon error, and testing for this.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-24  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (default_gdb_start): Handle eof.
	(clean_restart): Detect and handle gdb_start failure.  Return -1 upon
	failure.
	* gdb.base/readnever.exp: Handle clean_restart failure.