GPrivate: eliminate a malloc for pthread_key_t on most platforms

We don't know how big a pthread_key_t is, so we malloc() a big enough chunk of memory for it and store a pointer into the GPrivate struct.

It turns out, on Linux, pthread_key_t is just an int, so we could much easier just store it directly into the struct.

https://bugzilla.gnome.org/show_bug.cgi?id=737445 Fixes: #931 (closed)

Closes #931 (closed)