parasite-syscall: use the ptrace requests PTRACE_(GET|SET)REGSET to retrieve and set CPU registers

This patch introduces the routines ptrace_get_gpregs() and ptrace_set_gpregs()
that wrap the ptrace interface to get and set CPU registers respectively.
The motivation is to make the CRIU code be compatible with architectures that
don't support the PTRACE_GETREGS and PTRACE_SETREGS ptrace calls ---
the requests PTRACE_GETREGSET and PTRACE_SETREGSET are implemented instead.

Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>