[DynInst_API:] commit: Fix const'ness and documentation of PC's TLS interfaces

[Matthew LeGendre <legendre1@xxxxxxxx>]

Back in February I added interfaces to ProcControlAPI to access TLS 
variables.  A student here reported some minor interface and documentation 
problems that I'm now fixing:
- The Thread::getThreadLocalAddress() interface was not a const-function 
when it should have been.
- The documentation for Thread::getThreadLocalAddress(), 
Thread::writeThreadLocalMemory(), and Thread::readThreadLocalMemory() 
incorrectly listed these functions as non-const.  I've updated the 
documentation to list these as const functions.
-Matt