Fix tests using only-i686 to use the correct only-x86 directive

We translate `i686` to `x86` which means tests marked as `only-i686`
never ran. Update those tests to use `only-x86`.

src/test/ui/extern/extern-methods.rs

@@ -1,5 +1,5 @@
 // run-pass
-// only-i686
+// only-x86
 
 trait A {
     extern "fastcall" fn test1(i: i32);

src/test/ui/extern/extern-thiscall.rs

@@ -1,5 +1,5 @@
 // run-pass
-// only-i686
+// only-x86
 
 #![feature(abi_thiscall)]
 

src/test/ui/extern/extern-vectorcall.rs

@@ -1,7 +1,7 @@
 // run-pass
 // revisions: x64 x32
 // [x64]only-x86_64
-// [x32]only-i686
+// [x32]only-x86
 
 #![feature(abi_vectorcall)]