[runtime] Fix stack size detection on osx mavericks. Fixes #15890.
[runtime] Fix the osx mavericks stack size calculation in sgen too.
Remove the TLS data for a thread along with the other information when reusing a thread.

With the soft debugger attached and using GCD, calling mono_thread_attach before calling into Mono followed by mono_thread_detach when done, can/will result in thread_startup being called multiple times if the thread is reused by GCD. Previously, only the hash table information about the thread was cleared, but not the TLS debugger data.