Access to OS variables

[ExtendeD]

OS variables such as stdin/stdout currently cannot be accessed.

They should be declared in syscalls.h with a variable flag.

The swi handler should simply return the address in the caller mode's r0 when it reads such a flag.

OS.h should provide _SYSCALL_*() macros for this.

[ExtendeD]

 stdin/stdout/stderr addresses.

[ExtendeD]

_SYSCALL_OSVAR() is now defined, the stdin/stdout/stderr os variables still need to be defined.