Use -Ccodegen-units=1 to make issue-23458 test deterministic

The test case fails with either one error or two errors.
Use a single code generation unit to avoid nondeterminism.

Author: tmiasko

Diff for: src/test/ui/issues/issue-23458.rs

@@ -1,5 +1,5 @@
 #![feature(llvm_asm)]
-
+// compile-flags: -Ccodegen-units=1
 // build-fail
 // only-x86_64