libthr: prevent setcontext() from masking SIGTHR

[vangyzen]

Summary

__thr_setcontext() mistakenly tested for the presence of SIGCANCEL
in its local ucontext_t instead of the parameter. Therefore,
if a thread calls setcontext() with a context whose signal mask
contains SIGTHR (a.k.a. SIGCANCEL), that signal will be blocked,
preventing the thread from being cancelled or suspended.

Reported by: gcc 6.1 via RISC-V tinderbox

[kib]

lib/libthr/thread/thr_sig.c

740 ↗

(On Diff #28854)

Fix style of the next line as well, add ().