AK: Enable AK::SharedBuffer for all platforms

A future patch could do some MacOS specific things for
set_volatile/set_nonvolatile. For now, swap out the defined(__linux__)
branches for simple not __serenity__ branches.

AK/SharedBuffer.cpp

@@ -24,16 +24,13 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#if defined(__serenity__) || defined(__linux__)
-
 #include <AK/SharedBuffer.h>
-#    include <AK/kmalloc.h>
-#    include <Kernel/API/Syscall.h>
 #include <stdio.h>
 
 #if defined(__serenity__)
+#    include <Kernel/API/Syscall.h>
 #    include <serenity.h>
-#    elif defined(__linux__)
+#else
 #    include <AK/String.h>
 #    include <fcntl.h>
 #    include <sys/mman.h>
@@ -56,7 +53,7 @@ RefPtr<SharedBuffer> SharedBuffer::create_with_size(int size)
         perror("shbuf_create");
         return nullptr;
     }
-#    elif defined(__linux__)
+#else
     // Not atomic, so don't create shared buffers from many threads too hard under lagom.
     static unsigned g_shm_id = 0;
 
@@ -120,7 +117,7 @@ RefPtr<SharedBuffer> SharedBuffer::create_from_shbuf_id(int shbuf_id)
     void* data = shbuf_get(shbuf_id, &size);
     if (data == (void*)-1)
         return nullptr;
-#    elif defined(__linux__)
+#else
     int fd = shm_open(shbuf_shm_name(shbuf_id).characters(), O_RDWR, S_IRUSR | S_IWUSR);
     if (fd < 0) {
         perror("shm_open");
@@ -171,7 +168,7 @@ SharedBuffer::~SharedBuffer()
         if (rc < 0) {
             perror("shbuf_release");
         }
-#    elif defined(__linux__)
+#else
         if (munmap(reinterpret_cast<u8*>(m_data) - sizeof(size_t), m_size + sizeof(size_t)) < 0)
             perror("munmap");
         if (shm_unlink(shbuf_shm_name(m_shbuf_id).characters()) < 0)
@@ -211,5 +208,3 @@ bool SharedBuffer::set_nonvolatile()
 }
 
 }
-
-#endif

AK/SharedBuffer.h

@@ -26,8 +26,6 @@
 
 #pragma once
 
-#if defined(__serenity__) || defined(__linux__)
-
 #include <AK/RefCounted.h>
 #include <AK/RefPtr.h>
 
@@ -73,5 +71,3 @@ private:
 }
 
 using AK::SharedBuffer;
-
-#endif