[heap] fix external heap configuration (#6123)

The Instance::HeapCAlloc() method can not be used in constructors when
EXTERNAL_HEAP is enabled but works well when internal heap is
used. This is because the external heap methods are set by public
otHeapSetCAllocFree API after the OT instance is constructed.

This commit fixes this issue by having Instance::HeapCAlloc() using
otPlatCAlloc when EXTERNAL_HEAP is enabled. It also fixes build issues
when MULTIPLE_INSTANCE and EXTERNAL_HEAP are used together.

tests/unit/test_platform.cpp

@@ -94,7 +94,7 @@ bool sDiagMode = false;
 
 extern "C" {
 
-#if OPENTHREAD_CONFIG_MULTIPLE_INSTANCE_ENABLE
+#if OPENTHREAD_CONFIG_HEAP_EXTERNAL_ENABLE
 void *otPlatCAlloc(size_t aNum, size_t aSize)
 {
     return calloc(aNum, aSize);