linux-user/s390x: Create target_ptrace.h

Move target_psw_t to target_ptrace.h.  Note that abi_ulong already
has an attribute for 8-byte alignment, so there's no need to carry
another on target_psw_t.

Remove the target_pt_regs; add target_s390x_reg to target_ptrace.h,
which matches what is actually used.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>