Include llvm-as in llvm-tools-preview component

Including llvm-as adds the ability to include assembly language fragments
that can be inlined using LTO.

src/bootstrap/lib.rs

@@ -177,6 +177,7 @@ const LLVM_TOOLS: &[&str] = &[
     "llvm-size", // used to prints the size of the linker sections of a program
     "llvm-strip", // used to discard symbols from binary files to reduce their size
     "llvm-ar", // used for creating and modifying archive files
+    "llvm-as", // used to convert LLVM assembly to LLVM bitcode
     "llvm-dis", // used to disassemble LLVM bitcode
     "llc",     // used to compile LLVM bytecode
     "opt",     // used to optimize LLVM bytecode