x86: stop unconditionally clearing PSL_T on the trace trap.

We certainly should clear PSL_T when calling the SIGTRAP signal
handler, which is already done by all x86 sendsig(9) ABI code. On the
other hand, there is no obvious reason why PSL_T needs to be cleared
when returning from the signal handler. For instance, Linux allows
userspace to set PSL_T and keep tracing enabled for the desired
period. There are userspace programs which would use PSL_T if we make
it possible, for instance sbcl.

Remember if PSL_T was set by PT_STEP or PT_SETSTEP by mean of TDB_STEP
flag, and only clear it when the flag is set.

Discussed with: Ali Mashtizadeh
Reviewed by: jhb (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D15054