wasm executor compile error

[woodsmur]

llvm-tools-preview component should be installed before building substrate wasm executor, otherwise there'll be an error.

vagrant@ubuntu-cosmic:~/substrate$ ./scripts/build.sh
*** Building wasm binaries in substrate/executor/wasm
   Compiling runtime-test v0.1.0 (file:///home/vagrant/substrate/substrate/executor/wasm)
error: linking with `rust-lld` failed: exit code: 127
  |
  = note: "rust-lld" "-flavor" "wasm" "-L" "/home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib" "/home/vagrant/substrate/substrate/executor/wasm/target/wasm32-unknown-unknown/release/deps/runtime_test.runtime_test.3yru7t3n-cgu.3.rcgu.o" "-o" "/home/vagrant/substrate/substrate/executor/wasm/target/wasm32-unknown-unknown/release/deps/runtime_test.wasm" "--gc-sections" "-O3" "-L" "/home/vagrant/substrate/substrate/executor/wasm/target/wasm32-unknown-unknown/release/deps" "-L" "/home/vagrant/substrate/substrate/executor/wasm/target/release/deps" "-L" "/home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib" "/home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libcompiler_builtins-2a5c24d65378adf1.rlib" "--no-threads" "-z" "stack-size=1048576" "--stack-first" "--allow-undefined" "--no-entry" "--export-table"
  = note: rust-lld: error while loading shared libraries: libLLVM-7.so: cannot open shared object file: No such file or directory


error: aborting due to previous error

error: Could not compile `runtime-test`.

To learn more, run the command again with --verbose.

Running rustup component add llvm-tools-preview --toolchain=nightly will fix the compile error.

Here is the PR.