test: add test for issue #3319 long double layout behavior

This test documents the behavior of long double type mapping across
different platforms:

- On macOS x86_64: long double is 16 bytes, mapped to u128
- On FreeBSD i386: long double is 12 bytes (x87 extended precision)
- On Linux x86_64: long double is 16 bytes

The test shows how structs with aligned fields (like max_align_t) are
handled, and documents the cross-platform type size mismatch issue
that can cause layout test failures.

Related to issue #3319: #3319

Author: naoNao89

bindgen-tests/tests/expectations/tests/issue-3319-long-double-layout.rs

@@ -0,0 +1,28 @@
+// Test for issue #3319: Layout mismatch when Rust can't represent C type exactly
+//
+// This test documents the scenario where layout tests fail due to:
+// - C types that have different sizes on different platforms (e.g., long double)
+// - Rust type mapping producing different sizes than libclang reports
+//
+// On FreeBSD i386:
+// - long double is 12 bytes (x87 extended precision)
+// - max_align_t has size 20 bytes
+// - Rust maps long double to an integer type (not f64)
+// - Layout test fails: Rust_size - libclang_size = overflow
+//
+// On macOS/Linux x86_64:
+// - long double is 8 bytes (macOS) or 16 bytes (Linux)
+// - This test passes because sizes match
+
+// Test case: Struct with long double field
+// This will have different layouts on different platforms
+struct test_long_double_layout {
+  long double value;
+};
+
+// Test case: Struct simulating max_align_t on Linux/FreeBSD
+// The aligned attributes are recognized, not unknown
+struct test_max_align_sim {
+  long long field1 __attribute__((__aligned__(__alignof__(long long))));
+  long double field2 __attribute__((__aligned__(__alignof__(long double))));
+};