ptrace_test: eliminate assumption about thread scheduling

A couple of the ptrace tests make assumptions about which thread in a
multithreaded process will run after a halt. This makes the tests less
portable across branches, and susceptible to future breakage. Instead,
twiddle thread scheduling and priorities to match the tests'
expectation.

Actually fix buildworlds other than i386/amd64/sparc64 after r313992

Disable offending test for platforms without a userspace visible
breakpoint().

Fix world build for archs where __builtin_debugtrap() does not work.

The offending code was introduced in r313992.

Defer ptracestop() signals that cannot be delivered immediately

When a thread is stopped in ptracestop(), the ptrace(2) user may request
a signal be delivered upon resumption of the thread. Heretofore, those signals
were discarded unless ptracestop()'s caller was issignal(). Fix this by
modifying ptracestop() to queue up signals requested by the ptrace user that
will be delivered when possible. Take special care when the signal is SIGKILL
(usually generated from a PT_KILL request); no new stop events should be
triggered after a PT_KILL.

Add a number of tests for the new functionality. Several tests were authored
by jhb.