[Zephyr] Fix sys_heap malloc build with LTO

Fix the build of Matter samples with sys_heap malloc enabled.
The linker errors would occur because:
1. The realloc() function is not used outside the LTO-
   optimized code.
2. Thus, the LTO linker plugin would mark __wrap_realloc
   symbol as PREVAILING_DEF_IRONLY, which would enable
   the compiler to optimize away the function.
3. As a result, undefined references to realloc() could not
   be resolved by the linker.

Mark malloc/calloc/realloc/free as externally visible.

Signed-off-by: Damian Krolik <[email protected]>

src/platform/Zephyr/SysHeapMalloc.cpp

@@ -30,9 +30,6 @@ extern "C" {
 #include <cstdint>
 #include <cstring>
 
-// Construct name of the given function wrapped with the `--wrap=symbol` GCC option.
-#define WRAP(f) __wrap_##f
-
 using namespace chip;
 
 namespace {
@@ -99,7 +96,7 @@ void * Calloc(size_t num, size_t size)
         return nullptr;
     }
 
-    void * mem = malloc(totalSize);
+    void * mem = Malloc(totalSize);
 
     if (mem)
     {
@@ -156,27 +153,37 @@ void ResetMaxStats()
 
 extern "C" {
 
-void * WRAP(malloc)(size_t size) __attribute((alias("_ZN4chip11DeviceLayer6Malloc6MallocEj")));
-void * WRAP(calloc)(size_t num, size_t size) __attribute((alias("_ZN4chip11DeviceLayer6Malloc6CallocEjj")));
-void * WRAP(realloc)(void * mem, size_t size) __attribute((alias("_ZN4chip11DeviceLayer6Malloc7ReallocEPvj")));
-void WRAP(free)(void * mem) __attribute((alias("_ZN4chip11DeviceLayer6Malloc4FreeEPv")));
+// Construct the name of a function wrapped with the `--wrap=symbol` GCC option.
+#define WRAP(f) __wrap_##f
+
+// Define a function as an alias of another function.
+#define ALIAS(f) __attribute((alias(f)))
+
+// Mark a function as externally visible so that it is not optimized-away even
+// if LTO or whole-program optimization is enabled.
+#define EXTERNALLY_VISIBLE __attribute((externally_visible))
+
+EXTERNALLY_VISIBLE void * WRAP(malloc)(size_t size) ALIAS("_ZN4chip11DeviceLayer6Malloc6MallocEj");
+EXTERNALLY_VISIBLE void * WRAP(calloc)(size_t num, size_t size) ALIAS("_ZN4chip11DeviceLayer6Malloc6CallocEjj");
+EXTERNALLY_VISIBLE void * WRAP(realloc)(void * mem, size_t size) ALIAS("_ZN4chip11DeviceLayer6Malloc7ReallocEPvj");
+EXTERNALLY_VISIBLE void WRAP(free)(void * mem) ALIAS("_ZN4chip11DeviceLayer6Malloc4FreeEPv");
 
-void * WRAP(_malloc_r)(_reent *, size_t size)
+EXTERNALLY_VISIBLE void * WRAP(_malloc_r)(_reent *, size_t size)
 {
     return WRAP(malloc)(size);
 }
 
-void * WRAP(_calloc_r)(_reent *, size_t num, size_t size)
+EXTERNALLY_VISIBLE void * WRAP(_calloc_r)(_reent *, size_t num, size_t size)
 {
     return WRAP(calloc)(num, size);
 }
 
-void * WRAP(_realloc_r)(_reent *, void * mem, size_t size)
+EXTERNALLY_VISIBLE void * WRAP(_realloc_r)(_reent *, void * mem, size_t size)
 {
     return WRAP(realloc)(mem, size);
 }
 
-void WRAP(_free_r)(_reent *, void * mem)
+EXTERNALLY_VISIBLE void WRAP(_free_r)(_reent *, void * mem)
 {
     WRAP(free)(mem);
 }