Raise a proper SIGTRAP / TRAP_TRACE signal for a PT_STEP step on arm.

[jhb]

Summary

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.

Test Plan

• Run the ptrace_test kyua tests on arm. Previously the tests using PT_STEP would fail, now all tests pass.