PIOS Sensors: remove unneeded check

This broke sim compilation on OSX
FreeRTOS OSX: fixes for compilation on OSX 10.9

Various variables were unused. Clang has better static
checking.
Flight: remove old Stack_Change_Weak methods

This was created as a work around for simulation issues ages
ago but is no longer required and now causes compilation
errors.
SimOSX: fix for compilation issue

Do not try to require -lgcc
Sim: set max sensor rate to allow arming

This blocked arming in simulation modes.
Merge pull request #1023 from peabody124/sim_osx_fix

Fixes sim osx