[gdb/testsuite] Fix gdb.base/maint.exp with -readnow

With test-case gdb.base/maint.exp and target board -readnow, I run into:
...
FAIL: gdb.base/maint.exp: maint info line-table w/o a file name
...

The problem is that this and other regexps anchored using '^':
...
    -re "^$gdb_prompt $" {
...
don't trigger because other regexps don't consume the entire preceding line.

This is partly due to the addition of the IS-STMT column.

Fix this by making the regexps consume entire lines.

Tested on x86_64-linux with native and target board readnow, as well as
check-read1 and check-readmore.