Provide llvm-strip in llvm-tools component

Shipping this tool gives people reliable way to reduce the generated executable size.

I'm not sure if this strip tool is available from the llvm version current rust is built on. But let's take a look. @japaric

src/bootstrap/lib.rs

@@ -204,7 +204,8 @@ const LLVM_TOOLS: &[&str] = &[
     "llvm-objcopy", // used to transform ELFs into binary format which flashing tools consume
     "llvm-objdump", // used to disassemble programs
     "llvm-profdata", // used to inspect and merge files generated by profiles
-    "llvm-size", // prints the size of the linker sections of a program
+    "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
 ];
 
 /// A structure representing a Rust compiler.