Fix the machine-dependent ptrace requests to respect LWP number

Fix the machine-dependent ptrace register-related requests (e.g.
PT_GETXMMREGS, PT_GETXSTATE on x86) to correctly respect the LWP number
passed as the data argument.  Before this change, these requests
did not operate on the requested LWP of a multithreaded program.

This change required moving ptrace_update_lwp() out of unit scope,
and changing ptrace_machdep_dorequest() function to take a pointer
to pointer as the second argument, consistently with ptrace_regs().

I am planning to extend the ATF ptrace() register tests in the future
to check for regressions in multithreaded programs, as time permits.

Reviewed by kamil.

(mgorny)