[vm] Fix warnings in GCC 9.3.0 and XCode 12.0.

Bug: https://github.com/flutter/flutter/issues/66203
Change-Id: Ifff0c211e2ecdcb56d62c273a471fc05bbae9c37
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/161602
Reviewed-by: Chinmay Garde <chinmaygarde@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/include/dart_api_dl.c

@@ -16,7 +16,7 @@ DART_API_ALL_DL_SYMBOLS(DART_API_DL_DEFINITIONS)
 
 #undef DART_API_DL_DEFINITIONS
 
-typedef void (*DartApiEntry_function)();
+typedef void* DartApiEntry_function;
 
 DartApiEntry_function FindFunctionPointer(const DartApiEntry* entries,
                                           const char* name) {