Some applications may not call eglReleaseThread() before terminating a
thread, which can cause a minor memory leak. pthread_key_create()
supports specifying a destructor callback to implicitly free the
thread-local storage on thread termination. On Windows we use the
DLL_THREAD_DETACH signal to free it explicitly. This change also
simplifies TLS management by removing the ability to set a new value.

Bug swiftshader:80

Change-Id: I249ca519c1c9ab722e570516d0931cb17ea081bb
Reviewed-on: https://swiftshader-review.googlesource.com/12068Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>