Fix platform-specific ASGCT_CallFrame alignment and factor out COMMA macro

The dropped trace implementation was using inconsistent field initialization
that failed on platforms with different ASGCT_CallFrame layouts. Fixed by:

- Using LP64_ONLY(0 COMMA) pattern for proper 64-bit platform padding
- Factoring out COMMA macro definition to arch.h headers
- Simplifying dropped trace structure to match storage_overflow pattern

This resolves CI test failures in MetadataNormalisationTest and ContextCpuTest
while maintaining consistent cross-platform behavior.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jbachorik

ddprof-lib/src/main/cpp/callTraceHashTable.cpp

@@ -6,10 +6,9 @@
 #include "callTraceHashTable.h"
 #include "counters.h"
 #include "os.h"
+#include "arch.h" // For COMMA macro
 #include <string.h>
 
-#define COMMA ,
-
 static const u32 INITIAL_CAPACITY = 65536;
 static const u32 CALL_TRACE_CHUNK = 8 * 1024 * 1024;
 static const u64 OVERFLOW_TRACE_ID = 0x7fffffffffffffffULL;  // Max 64-bit signed value

ddprof-lib/src/main/cpp/callTraceStorage.cpp

@@ -8,6 +8,7 @@
 #include "counters.h"
 #include "common.h"
 #include "vmEntry.h" // For BCI_ERROR constant
+#include "arch.h" // For LP64_ONLY macro and COMMA macro
 #include <string.h>
 #include <atomic>
 
@@ -47,24 +48,10 @@ CallTraceStorage::~CallTraceStorage() {
 
 CallTrace* CallTraceStorage::getDroppedTrace() {
     // Static dropped trace object - created once and reused
-    static const char dropped_method_name[] = "<dropped due to contention>";
-    static struct {
-        CallTrace trace;
-        ASGCT_CallFrame frame; // Additional frame storage beyond the [1] in CallTrace
-    } dropped_trace_storage = {
-        .trace = {
-            .truncated = false,
-            .num_frames = 1,
-            .trace_id = DROPPED_TRACE_ID,
-            .frames = {{
-                .bci = BCI_ERROR, // Mark as native frame with error string
-                .method_id = (jmethodID)dropped_method_name // String name for native frame
-            }}
-        },
-        .frame = {} // Unused but needed for proper alignment
-    };
+    // Use same pattern as storage_overflow trace for consistent platform handling
+    static CallTrace dropped_trace = {false, 1, DROPPED_TRACE_ID, {BCI_ERROR, LP64_ONLY(0 COMMA) (jmethodID)"<dropped due to contention>"}};
 
-    return &dropped_trace_storage.trace;
+    return &dropped_trace;
 }
 
 void CallTraceStorage::registerLivenessChecker(LivenessChecker checker) {