Add ignore-cross-compile directive for compiletest

Loadable syntax extensions don't work when cross compiling (see #12102), so the
fourcc tests all need to be ignored. They're valuable tests, so they shouldn't
be outright ignored, so they're now flagged with ignore-cross-compile

src/test/compile-fail/syntax-extension-fourcc-bad-len.rs

@@ -10,7 +10,7 @@
 
 // ignore-stage1
 // ignore-pretty
-// ignore-android
+// ignore-cross-compile #12102
 
 #[feature(phase)];
 

src/test/compile-fail/syntax-extension-fourcc-invalid-endian.rs

@@ -10,7 +10,7 @@
 
 // ignore-stage1
 // ignore-pretty
-// ignore-android
+// ignore-cross-compile #12102
 
 #[feature(phase)];
 

src/test/compile-fail/syntax-extension-fourcc-non-ascii-str.rs

@@ -10,7 +10,7 @@
 
 // ignore-stage1
 // ignore-pretty
-// ignore-android
+// ignore-cross-compile #12102
 
 #[feature(phase)];
 

src/test/compile-fail/syntax-extension-fourcc-non-literal.rs

@@ -10,7 +10,7 @@
 
 // ignore-stage1
 // ignore-pretty
-// ignore-android
+// ignore-cross-compile #12102
 
 #[feature(phase)];
 

src/test/compile-fail/syntax-extension-fourcc-unsupported-literal.rs

@@ -10,7 +10,7 @@
 
 // ignore-stage1
 // ignore-pretty
-// ignore-android
+// ignore-cross-compile #12102
 
 #[feature(phase)];