Simplify signal handling code in libthr by removing use of SYS_sigreturn

The use of SYS_sigreturn is unnecessary here.

If handle_signal is called when a signal is delivered, it can just
return normally back to sigcode which will call sigreturn anyway.

In case handle_signal is called by check_deferred_signal, using
setcontext is better than SYS_sigreturn because that is the correct
system call to pair with the earlier getcontext.

Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D44893