[gdb] Mention --with/without-system-readline for --configuration

[gdb] Mention --with/without-system-readline for --configuration

Simon reported that the new test-case gdb.tui/pr30056.exp fails with system
readline.

This is because the test-case requires a fix in readline that's present in our
in-repo copy of readline, but most likely not in any system readline yet.

Fix this by:
- mentioning --with-system-readline or --without-system-readline in the
  configuration string.
- adding a new proc with_system_readline that makes this information available
  in the testsuite.
- using this in test-case gdb.tui/pr30056.exp to declare it unsupported for
  --with-system-readline.

Tested on x86_64-linux.

Reported-By: Simon Marchi <simon.marchi@efficios.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>