Reimplement bdf0f24bb16d556a5b by checking for the caller' ABI in
the implementation of PT_GET_SC_ARGS, and copying out everything if
it is Linuxolator.
Also fix a minor information leak: if PT_GET_SC_ARGS_ALL is done on the
thread reused after other process, it allows to read some number of that
thread last syscall arguments. Clear td_sa.args in thread_alloc().
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D31968
f575573ca5