parasite-syscall.c: introduced the macros REG_RES and REG_IP

The macro REG_RES is to be used to access the syscall return value register
in the struct user_regs_struct_t.

The macro REG_IP is to be used to access the program counter register
in the struct user_regs_struct_t.

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