Add bootstrap recipe.

recipes/bootstrap/recipe.toml

+[recipe]
+[build]
+template = "custom"
+script = """
+ARCH="$(echo "${TARGET}" | cut -d - -f1)"
+nasm -o "${COOKBOOK_BUILD}/asm.o" -f elf64 "${COOKBOOK_SOURCE}/src/${ARCH}.asm"
+cargo -Zbuild-std=core,alloc,compiler_builtins -Zbuild-std-features=compiler-builtins-mem rustc --target "${TARGET}" --manifest-path "${COOKBOOK_SOURCE}/Cargo.toml" --release --target-dir "${COOKBOOK_BUILD}" -- --emit obj="${COOKBOOK_BUILD}/${TARGET}/release/libbootstrap.a"
+compiler_builtins=`find "${COOKBOOK_BUILD}/x86_64-unknown-redox/release/deps/" -name "*libcompiler_builtins*.rlib"`
+ld -o "${COOKBOOK_STAGE}/bootstrap" --gc-sections -T "${COOKBOOK_SOURCE}/src/${ARCH}.ld" "${COOKBOOK_BUILD}/asm.o" "${COOKBOOK_BUILD}/${TARGET}/release/libbootstrap.a" "${COOKBOOK_SYSROOT}/lib/libc.a" "$compiler_builtins"
+"""
+dependencies = ["relibc"]