Kernel: Add Syscalls/fork.cpp to aarch64 build

Kernel/CMakeLists.txt

@@ -276,6 +276,7 @@ set(KERNEL_SOURCES
     Syscalls/faccessat.cpp
     Syscalls/fallocate.cpp
     Syscalls/fcntl.cpp
+    Syscalls/fork.cpp
     Syscalls/fsync.cpp
     Syscalls/ftruncate.cpp
     Syscalls/futex.cpp
@@ -402,7 +403,6 @@ if ("${SERENITY_ARCH}" STREQUAL "x86_64")
         Panic.cpp
         Syscall.cpp
         Syscalls/execve.cpp
-        Syscalls/fork.cpp
     )
 
     set(KERNEL_SOURCES

Kernel/Syscalls/fork.cpp

@@ -125,6 +125,9 @@ ErrorOr<FlatPtr> Process::sys$fork(RegisterState& regs)
 
     dbgln_if(FORK_DEBUG, "fork: child will begin executing at {:#04x}:{:p} with stack {:p}, kstack {:p}",
         child_regs.cs, child_regs.rip, child_regs.rsp, child_regs.rsp0);
+#elif ARCH(AARCH64)
+    (void)regs;
+    TODO_AARCH64();
 #else
 #    error Unknown architecture
 #endif