[lldb] Support gdbserver signals

GDB and LLDB use different signal models.  GDB uses a predefined set
of signal codes, and maps platform's signos to them.  On the other hand,
LLDB has historically simply passed native signos.

In order to improve compatibility between LLDB and gdbserver, the GDB
signal model should be used.  However, GDB does not provide a mapping
for all existing signals on Linux and unsupported signals are passed
as 'unknown'.  Limiting LLDB to this behavior could be considered
a regression.

To get the best of both worlds, use the LLDB signal model when talking
to lldb-server, and the GDB signal model otherwise.  For this purpose,
new versions of lldb-server indicate "native-signals+" via qSupported.
At the same time, we also detect older versions of lldb-server
via QThreadSuffixSupported for backwards compatibility.  If neither test
succeeds, we assume gdbserver or another implementation using GDB model.

Differential Revision: https://reviews.llvm.org/D108078