Restrict test to x86_64.

(cherry picked from commit 5529e2f)

Author: cjgillot

tests/codegen/sroa-fragment-debuginfo.rs

@@ -1,5 +1,8 @@
 // compile-flags: -g -Zmir-opt-level=0 -Zmir-enable-passes=+ScalarReplacementOfAggregates
 // compile-flags: -Cno-prepopulate-passes
+//
+// Tested offsets are only correct for x86_64.
+// only-x86_64
 
 #![crate_type = "lib"]