[gdb/cli] Handle pending ^C after rl_callback_read_char for readline 7

In commit faf01aee ("[gdb] Handle pending ^C after rl_callback_read_char")
we handled a problem (described in detail in that commit) for readline >= 8
using public readline functions rl_pending_signal and rl_check_signals.

For readline 7 (note that we require at least readline 7 so there's no need to
worry about readline 6), there was no fix though, because rl_check_signals was
not available.

Fix this by instead using the private readline function _rl_signal_handler.

There is precedent for using private readline variables and functions, but
it's something we want to get rid of (PR build/10723).  Nevertheless, I think
we can allow this specific instance because it's not used when building
against readline >= 8.

[ In the meanwhile, a fix was committed in the devel branch of the readline
repo, contained in commit 8d0c439 ("rollup of changes since readline-8.2"),
first proposed here (
https://lists.gnu.org/archive/html/bug-readline/2022-10/msg00008.html ). ]

Tested on x86_64-linux, against system readline 7.0 on openSUSE Leap 15.4.

PR cli/27813
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27813