Use a single codegen unit to reduce non-determinism in srcloc.rs test

When building with multiple codegen units the test case can fail with
only a subset of all errors. Use a single codegen unit as a workaround.

src/test/ui/asm/srcloc.rs

 // no-system-llvm
 // only-x86_64
 // build-fail
-
+// compile-flags: -Ccodegen-units=1
 #![feature(asm)]
 
 // Checks that inline asm errors are mapped to the correct line in the source code.