gdb/testsuite: get_set_option_choices: expect \r\n after each item

gdb/testsuite: get_set_option_choices: expect \r\n after each item

I get some random failures since commit 8d45c3a82a0e ("[gdb/testsuite]
Set completions to unlimited in get_set_option_choices"), which can be
reproduced with:

    $ make check-read1 TESTS="gdb.base/parse_number.exp"

For instance:

    set architecture A^M
    Ambiguous item "A".^M
    (gdb) FAIL: gdb.base/parse_number.exp: arch=A: set architecture A

The problem is the regexp in get_set_option_choices, it is possible that
is only matches part of a completion result.  With check-read1, that is
always one letter.

Fix this by expecting the \r\n at the end of the line, so we only match
entire results.  Use ^ in match patterns to ensure we don't miss any
output.

Approved-By: Tom de Vries <tdevries@suse.de>
Change-Id: Ib1733737feab7dde0f7095866e089081a891054e