Do not run rustfmt on the compiler-builtins subtree

This will get checked in the compiler-builtins repository. The rules are
slightly different from rust-lang/rust since `use_small_heuristics =
"Max"` tends to create difficult to read output with math-heavy code
(e.g.  multiple `if` statements with numbers are small enough that they
sometimes get collapsed into a single line).

rustfmt.toml

@@ -29,6 +29,7 @@ ignore = [
 
     # Do not format submodules.
     "library/backtrace",
+    "library/compiler-builtins",
     "library/portable-simd",
     "library/stdarch",
     "src/doc/book",