compat32: Translate userland PT_* request values into kernel codes

Currently, the compat32 passes PT_* request values to kernel functions
without translation.  This works fine for low PT_* requests that happen
to have the same values both on i386 and amd64.  However, for requests
higher than PT_SETFPREGS, the value passed from userland (matching i386
const) does not match the correct kernel (amd64) request.  As a result,
e.g. when compat32 process calls PT_GETDBREGS, kernel actually processes
it as PT_SETSTEP.

To resolve this, introduce support for compat32 PT_* request
translation.  The interface is based on PTRACE_TRANSLATE_REQUEST32 macro
that is defined to a mapping function on architectures needing it.
In case of amd64, this function maps userland i386 PT_* values into
appropriate amd64 PT_* values.

For the time being, the two additional PT_GETXMMREGS and PT_SETXMMREGS
requests are unsupported due to lack of matching free amd64 constant.

(mgorny)