Everywhere: Add RISC-V 64 target to the build system

This is a minimal set of changes to allow `serenity.sh build riscv64` to
successfully generate the build environment and start building. This
includes some, but not all, assembly stubs that will be needed later on;
they are currently empty.

Meta/ShellCompletions/zsh/_serenity

@@ -46,6 +46,7 @@ _serenity() {
     targets=(
         'x86_64:Target x86_64 or $SERENITY_ARCH (default)'
         'aarch64:Target aarch64'
+        'riscv64:Target riscv64'
         'lagom:Target host machine'
     )