Use LLDB debugger for Android (GDB will be removed from NDK)

GDB support will be removed from the NDK with the next release r24, which according to their release schedule should be released in Fall 2021. At that point, Qt Creator will need support for LLDB to continue supporting debugging of Android apps using the latest NDK out of the box.

From the NDK release notes:

Support for GDB has ended. GDB will be removed from the next release. Use LLDB instead. Note that ndk-gdb uses LLDB by default.

LLDB support was implemented internally as part of QTCREATORBUG-23260, but afaik is not available publicly in the latest Qt Creator releases.