Stop single stepping in signal handers on arm64

[andrew]

Summary

We should clear the single step flag when entering a signal hander and
set it when returning. This fixes the ptrace__PT_STEP_with_signal test.

While here add support for userspace to set the single step bit as on
x86. This can be used by userspace for self tracing.

[andrew]

64-bit works. I still need to test the COMPAT32 case.

[kib]

tests/sys/kern/basic_signal.c

93

This should be

ucp->uc_mcontext.mc_rflags |= PSL_T;

for amd64, and

ucp->uc_mcontext.mc_eflags |= PSL_T;

for i386.

[andrew]

Add COMPAT32 support

[andrew]

Asd untested x86 test support