Properly account signal count in the explicit-reply path (when source-
client gets immediately unblocked by 'signal_session_component::submit').
This patch prevents the delivery of superfluous signals with num == 0.
- Let hybrid Linux/Genode programs use POSIX threads for the
implementation of the Thread API.
- Prevent linkage of cxx library to hybrid Linux/Genode programs because
the cxx functionality is covered by glibc.
