Prekernel: Make build on aarch64

Add a dummy Arch/aarch64/boot.S that for now does nothing but
let all processor cores sleep.

For now, none of the actual Prekernel code is built for aarch64.

Kernel/CMakeLists.txt

@@ -516,7 +516,5 @@ install(FILES "${CMAKE_CURRENT_BINARY_DIR}/kernel.map" DESTINATION res)
 serenity_install_headers(Kernel)
 serenity_install_sources(Kernel)
 
-if (NOT "${SERENITY_ARCH}" STREQUAL "aarch64")
-    add_subdirectory(Prekernel)
-endif()
+add_subdirectory(Prekernel)
 add_subdirectory(Modules)

Kernel/Prekernel/Arch/aarch64/boot.S

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2021, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+.text
+
+.global start
+.type start, @function
+start:
+  b Lhalt
+
+Lhalt:
+  wfi
+  b Lhalt

Kernel/Prekernel/CMakeLists.txt

@@ -1,16 +1,30 @@
 set(SOURCES
-    Arch/x86/boot.S
-    Arch/x86/multiboot.S
-    init.cpp
-    UBSanitizer.cpp
-    ../MiniStdLib.cpp
-    ../../Userland/Libraries/LibELF/Relocation.cpp
 )
+if ("${SERENITY_ARCH}" STREQUAL "aarch64")
+    set(SOURCES
+        ${SOURCES}
+        Arch/aarch64/boot.S
+    )
+else()
+    set(SOURCES
+        ${SOURCES}
+        Arch/x86/boot.S
+        Arch/x86/multiboot.S
+        # FIXME: Eventually, some of these should build on aarch64 too.
+        UBSanitizer.cpp
+        init.cpp
+        ../MiniStdLib.cpp
+        ../../Userland/Libraries/LibELF/Relocation.cpp
+    )
+endif()
+
 
 if ("${SERENITY_ARCH}" STREQUAL "i686")
     set(PREKERNEL_TARGET Prekernel32)
-else()
+elseif ("${SERENITY_ARCH}" STREQUAL "x86_64")
     set(PREKERNEL_TARGET Prekernel64)
+else()
+    set(PREKERNEL_TARGET Prekernel)
 endif()
 
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -static")
@@ -27,11 +41,13 @@ else()
     target_link_libraries(${PREKERNEL_TARGET} gcc supc++)
 endif()
 
-add_custom_command(
-    TARGET ${PREKERNEL_TARGET} POST_BUILD
-    COMMAND ${CMAKE_OBJCOPY} -O elf32-i386 ${CMAKE_CURRENT_BINARY_DIR}/${PREKERNEL_TARGET} ${CMAKE_CURRENT_BINARY_DIR}/Prekernel
-    BYPRODUCTS ${CMAKE_CURRENT_BINARY_DIR}/Prekernel
-)
+if ("${SERENITY_ARCH}" STREQUAL "i686" OR "${SERENITY_ARCH}" STREQUAL "x86_64")
+    add_custom_command(
+        TARGET ${PREKERNEL_TARGET} POST_BUILD
+        COMMAND ${CMAKE_OBJCOPY} -O elf32-i386 ${CMAKE_CURRENT_BINARY_DIR}/${PREKERNEL_TARGET} ${CMAKE_CURRENT_BINARY_DIR}/Prekernel
+        BYPRODUCTS ${CMAKE_CURRENT_BINARY_DIR}/Prekernel
+    )
+endif()
 
 install(FILES "${CMAKE_CURRENT_BINARY_DIR}/Prekernel" DESTINATION boot)