Kernel: Split ScopedCritical so header is platform independent

A new header file has been created in the Arch/ folder while the
implementation has been moved into a CPP living in the X86 folder.

Kernel/CMakeLists.txt

@@ -288,6 +288,11 @@ set(KERNEL_SOURCES
 )
 
 if ("${SERENITY_ARCH}" STREQUAL "i686" OR "${SERENITY_ARCH}" STREQUAL "x86_64")
+    set(KERNEL_SOURCES
+        ${KERNEL_SOURCES}
+        Arch/x86/ScopedCritical.cpp
+    )
+
     set(KERNEL_SOURCES
         ${KERNEL_SOURCES}
         ${CMAKE_CURRENT_SOURCE_DIR}/Arch/x86/${KERNEL_ARCH}/ASM_wrapper.cpp
@@ -361,7 +366,11 @@ if (NOT "${SERENITY_ARCH}" STREQUAL "aarch64")
 else()
     set(SOURCES
         ${SOURCES}
+        ${AK_SOURCES}
+        
         Arch/aarch64/dummy.cpp
+        Arch/aarch64/ScopedCritical.cpp
+        
         UBSanitizer.cpp
     )