This fixes an inconsistency since
fed7e7a7 (ntdll: Simulate a syscall
return when starting a thread); in that commit, on i386 and
x86_64, the syscall_frame used for starting the thread was
zeroed with a memset, while it wasn't on arm and arm64.

This had a noticable effect on float maths, where the
uninitialized frame->restore_flags field could have the
CONTEXT_FLOATING_POINT bit set. (call_init_thunk only ORs in
CONTEXT_INTEGER into the restore_flags field, on all architectures).
If that bit was set, it would restore uninitialized data into the
fpscr register, possibly setting the FPU in a nondefault rounding
mode.
Signed-off-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>