Kernel/riscv64: Add assembly trap handler

This trap handler can't handle traps from userspace yet.

Kernel/CMakeLists.txt

@@ -545,6 +545,7 @@ elseif("${SERENITY_ARCH}" STREQUAL "riscv64")
         Arch/riscv64/SBI.cpp
         Arch/riscv64/SmapDisabler.cpp
         Arch/riscv64/Timer.cpp
+        Arch/riscv64/trap_handler.S
     )
 
     # NOTE: These files cannot use a stack protector and sanitizers, as these will cause accesses to global variables to be inserted