Add support for BSD systems

In file included from webrtc/rtc_base/platform_thread_types.cc:11:
webrtc/rtc_base/platform_thread_types.h:47:1: error: unknown type name 'PlatformThreadId'
PlatformThreadId CurrentThreadId();
^
webrtc/rtc_base/platform_thread_types.h:52:1: error: unknown type name 'PlatformThreadRef'
PlatformThreadRef CurrentThreadRef();
^
webrtc/rtc_base/platform_thread_types.h:55:29: error: unknown type name 'PlatformThreadRef'
bool IsThreadRefEqual(const PlatformThreadRef& a, const PlatformThreadRef& b);
                            ^
webrtc/rtc_base/platform_thread_types.h:55:57: error: unknown type name 'PlatformThreadRef'
bool IsThreadRefEqual(const PlatformThreadRef& a, const PlatformThreadRef& b);
                                                        ^
webrtc/rtc_base/platform_thread_types.cc:30:1: error: unknown type name 'PlatformThreadId'
PlatformThreadId CurrentThreadId() {
^
webrtc/rtc_base/platform_thread_types.cc:51:1: error: unknown type name 'PlatformThreadRef'
PlatformThreadRef CurrentThreadRef() {
^
webrtc/rtc_base/platform_thread_types.cc:61:29: error: unknown type name 'PlatformThreadRef'
bool IsThreadRefEqual(const PlatformThreadRef& a, const PlatformThreadRef& b) {
                            ^
webrtc/rtc_base/platform_thread_types.cc:61:57: error: unknown type name 'PlatformThreadRef'
bool IsThreadRefEqual(const PlatformThreadRef& a, const PlatformThreadRef& b) {
                                                        ^
webrtc/rtc_base/time_utils.cc:194:2: error: Unsupported platform.
 #error Unsupported platform.
  ^
In file included from webrtc/rtc_base/platform_thread.cc:11:
In file included from webrtc/rtc_base/platform_thread.h:22:
In file included from webrtc/rtc_base/thread_checker.h:17:
In file included from webrtc/rtc_base/synchronization/sequence_checker.h:17:
webrtc/rtc_base/synchronization/mutex.h:28:2: error: Unsupported platform.
 #error Unsupported platform.
  ^
webrtc/rtc_base/synchronization/mutex.h:52:3: error: unknown type name 'MutexImpl'
  MutexImpl impl_;
  ^
4 jobs for freebsd in 1 minute and 7 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Container
passed #6084674
aarch64
build-container-aarch64

00:00:21

passed #6084673
build-container-x86_64

00:00:17

 
  Build
passed #6084676
aarch64
build-aarch64

00:00:44

passed #6084675
build-x86_64

00:00:39