[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/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"