Previously, a step by PT_STEP resulted in no signal being raised to
the debugger so that a step was silently completed with the program
continuing to execute after the step. Fix by raising a SIGTRAP
signal with TRAP_TRACE as the signal code.
To simplify the error handling cases (if ptrace_clear_single_step()
fails, etc.) move the handling of PTRACE_BREAKPOINT into the
gdb_trapper() function. If ptrace_clear_single_step() fails,
gdb_trapper() won't claim the fault, and the default case of
SIGILL / ILL_OPC will be used.
Differential Revision: https://reviews.freebsd.org/D16100
433390a0ca