rtc_base: define PlatformThread{Id,Ref} on OpenBSD

pthread_self(3) returns a phtread_t (aka. pthread *) value.

With @ilya-fedin:  Redefine thread IDs as `intptr_t` on POSIX platforms
unless platform specific functions/types exist so as to avoid the
following warning on OpenBSD/amd64 7.0 using clang 11.1.0:
```
platform_thread_types.cc:57:10: error: cast from pointer to smaller type 'rtc::PlatformThreadId' (aka 'int') loses information
  return reinterpret_cast<PlatformThreadId>(pthread_self());
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           1 error generated.
```

At least Solaris defines `pid_t` as `long`, i.e. that warning would not
occur.

OpenBSD defines `pid_t` as `int`.

Linux uses its non-portable gettid(3) returing a `pid_t` value,
so stick with that.

src/rtc_base/platform_thread_types.cc

@@ -27,6 +27,8 @@ typedef HRESULT(WINAPI* RTC_SetThreadDescription)(HANDLE hThread,
 
 #if defined(WEBRTC_FREEBSD)
 #include <pthread_np.h>
+#elif defined(WEBRTC_OPENBSD)
+#include <pthread_np.h>
 #endif
 
 namespace rtc {
@@ -115,6 +117,8 @@ void SetCurrentThreadName(const char* name) {
   prctl(PR_SET_NAME, reinterpret_cast<unsigned long>(name));  // NOLINT
 #elif defined(WEBRTC_FREEBSD)
   pthread_setname_np(pthread_self(), name);
+#elif defined(WEBRTC_OPENBSD)
+  pthread_set_name_np(pthread_self(), name);
 #elif defined(WEBRTC_MAC) || defined(WEBRTC_IOS)
   pthread_setname_np(name);
 #endif

src/rtc_base/platform_thread_types.h

@@ -39,7 +39,13 @@ typedef DWORD PlatformThreadRef;
 typedef zx_handle_t PlatformThreadId;
 typedef zx_handle_t PlatformThreadRef;
 #elif defined(WEBRTC_POSIX)
+#if defined(WEBRTC_MAC) || defined(WEBRTC_IOS)
+typedef mach_port_t PlatformThreadId;
+#elif defined(WEBRTC_LINUX)
 typedef pid_t PlatformThreadId;
+#else
+typedef intptr_t PlatformThreadId;
+#endif
 typedef pthread_t PlatformThreadRef;
 #endif