Implement runtime suspension for OSX

Implement runtime suspension for OSX

This change implements runtime suspension for OSX using activation
injection, the same mechanism as Linux uses.
I have modified the activation injection mechanism by adding a new
function that CoreCLR registers with PAL and PAL calls it to check
if an instruction address is safe for injection. The injection is
performed only if this new function returns true.
The activation on OSX is performed by suspending the target thread,
checking the instruction address in the suspended thread's context
using the above mentioned function and modifying the context to
perform the injection only at injection safe place.