[gdb/testsuite] Make gdb.base/annota1.exp more robust

[gdb/testsuite] Make gdb.base/annota1.exp more robust

On openSUSE Tumbleweed I run into:
...
FAIL: gdb.base/annota1.exp: run until main breakpoint (timeout)
...

The problem is that the libthread_db message occurs at a location where it's
not expected:
...
Starting program: outputs/gdb.base/annota1/annota1 ^M
^M
^Z^Zstarting^M
^M
^Z^Zframes-invalid^M
[Thread debugging using libthread_db enabled]^M
Using host libthread_db library "/lib64/libthread_db.so.1".^M
^M
^Z^Zbreakpoints-invalid^M
^M
...

Fix this by making the matching more robust:
- rewrite the regexp such that each annotation is on a single line,
  starting with \r\n\032\032 and ending with \r\n
- add a regexp variable optional_re, that matches all possible optional
  output, and use it as a separator in the first part of the regexp

Tested on x86_64-linux.