Fix TLS for platforms without library constructors.

Fix TLS for platforms without library constructors.

Some platforms don't support constructors that get called when the
shared library gets loaded. This results in the thread-local-storage
key to never be allocated. Instead we can check if it was allocated on
each use. Also prevent creating new 'current' state if it already
exists, and avoid pthreads undefined behavior.

Bug b/25629882

Change-Id: I5e2486f88185150b976d88d0144e4d13a349eb79
Reviewed-on: https://swiftshader-review.googlesource.com/10451
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>