SYS funcs and GCC12 fixes (#1186)

* SYS functions and GCC 12 fixes

* Add implementation of _gettimeofday and settimeofday, _times
* Remove some GCC warnings about unimplemented SYS functions (e.g. _open) by making weak implementations that return errors.
* Removed _exit from crt0.S since we have a weak version in runtime.c and we don't want two weak impls since the linker can't pick. If the user omits runtime.c then they'll need to provide _exit or get the error
* Add sys/time.h to arch/cc.h for lwIP as it seems under GCC12 this is not getting included

src/rp2_common/pico_standard_link/crt0.S

@@ -263,10 +263,6 @@ platform_entry: // symbol for stack traces
     blx r1
     // exit should not return.  If it does, hang the core.
     // (fall thru into our hang _exit impl
-.weak _exit
-.type _exit,%function
-.thumb_func
-_exit:
 1: // separate label because _exit can be moved out of branch range
     bkpt #0
     b 1b