Fix: sizeof(pid_t) != sizeof(pthread_t)

[timpalpant]

The real-time profiling code manages an array of threads to profile. This array is an array of pthread_t identifiers, but the code to allocate the array uses sizeof(pid_t), which many not equal sizeof(pthread_t). In particular, on Linux 4.15 x86_64, sizeof(pthread_t) = 8 while sizeof(pid_t) = 4. This leads to crashes since the code will write beyond its allocated space.

Based on git history, I believe this has existed since the introduction of real_time support in ccae9f7.

Fixes #210.

[timpalpant]

@arigo if you don't mind taking a look, I'd appreciate it, or I can send another email to pypy-dev.

[arigo]

Looks like an obvious bug, yes.