Revert "[builtins] Fix union initialization"

This reverts commit 0d14ad57.

Reason for revert: Seems to add a static initializer:
https://build.chromium.org/p/client.v8/builders/V8%20Linux/builds/19779

Original change's description:
> [builtins] Fix union initialization
> 
> Fix d4c157ee commit
> 
> On big endian platforms wrong byte is being read when accessing
> "parameter_count" field of union, in function
> failing on MIPS big endian platforms.
> 
> Builtins: :GetStackParameterCount. It is the reason of several tests
> Bug: 
> Change-Id: Ia21aba80a89215022ee218d27552b5b4ea18a0db
> Reviewed-on: https://chromium-review.googlesource.com/603808
> Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#47330}

TBR=jkummerow@chromium.org,jarin@chromium.org,jgruber@chromium.org,predrag.rudic@imgtec.com,ivica.bogosavljevic@imgtec.com

Change-Id: I596bb1a04c72c8f5cb4f7b4a8e9161bd5889683d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/612287Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47331}

src/builtins/builtins.cc

@@ -26,11 +26,7 @@ namespace {
 struct BuiltinMetadata {
   const char* name;
   Builtins::Kind kind;
-  union KindSpecificData {
-    KindSpecificData(Address cpp_entry) : cpp_entry(cpp_entry) {}
-    KindSpecificData(int8_t parameter_count)
-        : parameter_count(parameter_count) {}
-    KindSpecificData() = default;
+  union {
     Address cpp_entry;       // For CPP and API builtins.
     int8_t parameter_count;  // For TFJ builtins.
   } kind_specific_data;
@@ -42,7 +38,7 @@ struct BuiltinMetadata {
 #define DECL_API(Name, ...) { #Name, Builtins::API, \
                               { FUNCTION_ADDR(Builtin_##Name) }},
 #define DECL_TFJ(Name, Count, ...) { #Name, Builtins::TFJ, \
-                              { static_cast<int8_t>(Count) }},
+                                     { reinterpret_cast<Address>(Count) }},
 #define DECL_TFC(Name, ...) { #Name, Builtins::TFC, {} },
 #define DECL_TFS(Name, ...) { #Name, Builtins::TFS, {} },
 #define DECL_TFH(Name, ...) { #Name, Builtins::TFH, {} },