gdb: fix for gdb.base/eof-exit.exp test failures

This fix relates to PR gdb/29032, this makes the test more stable by
ensuring that the Ctrl-D is only sent once the prompt has been
displayed.  This issue was also discussed on the mailing list here:

  https://sourceware.org/pipermail/gdb-patches/2022-April/187670.html

The problem identified in the bug report is that sometimes the Ctrl-D
(that the test sends to GDB) arrives while GDB is processing a
command.  When this happens the Ctrl-D is handled differently than if
the Ctrl-D is sent while GDB is waiting for input at a prompt.

The original intent of the test was that the Ctrl-D be sent while GDB
was waiting at a prompt, and that is the case the occurs most often,
but, when the Ctrl-D arrives during command processing, then GDB will
ignore the Ctrl-D, and the test will fail.

This commit ensures the Ctrl-D is always sent while GDB is waiting at
a prompt, which makes this test stable.

But, that still leaves an open question, what should happen when the
Ctrl-D arrives while GDB is processing a command?  This commit doesn't
attempt to answer that question, which is while bug PR gdb/29032 will
not be closed once this commit is merged.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29032

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