[Build] Generalize POSIX platform guards

Most non-Windows code paths use standard POSIX APIs (sockets, signals,
dlopen, threading) that work on any POSIX platform. Change Linux-specific
guards to !WIN32 or #else where the code is portable. Linux-specific
APIs (SIGRTMIN, Vulkan/X11, fontconfig/GTK) remain Linux-guarded.

src/xenia/base/atomic.h

@@ -74,7 +74,7 @@ inline void atomic_store_release(uint32_t new_value, volatile uint32_t* value) {
   _InterlockedExchange(reinterpret_cast<volatile long*>(value), new_value);
 }
 
-#elif XE_PLATFORM_LINUX || XE_PLATFORM_MAC
+#else
 
 inline int32_t atomic_inc(volatile int32_t* value) {
   return __sync_add_and_fetch(value, 1);
@@ -133,10 +133,6 @@ inline bool atomic_cas(int64_t old_value, int64_t new_value,
       reinterpret_cast<volatile int64_t*>(value), old_value, new_value);
 }
 
-#else
-
-#error No atomic primitives defined for this platform/cpu combination.
-
 #endif  // XE_PLATFORM
 
 inline uint32_t atomic_inc(volatile uint32_t* value) {

src/xenia/base/mutex.h

@@ -15,7 +15,7 @@
 #include "platform.h"
 #if XE_PLATFORM_WIN32
 #include "platform_win.h"
-#elif XE_PLATFORM_LINUX
+#else
 #include <sys/types.h>
 #endif
 #include "memory.h"