Fix implementation of bit count functions.

include/v8config.h

Index: include/v8config.h
diff --git a/include/v8config.h b/include/v8config.h
index 452ffc73e701066016813a30709f745b0d1cb52d..be02b6e6ea1cabfae0a48188cfc5a49560521e8b 100644
--- a/include/v8config.h
+++ b/include/v8config.h
@@ -175,7 +175,10 @@
 //  V8_HAS_ATTRIBUTE_VISIBILITY         - __attribute__((visibility)) supported
 //  V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT - __attribute__((warn_unused_result))
 //                                        supported
+//  V8_HAS_BUILTIN_CLZ                  - __builtin_clz() supported
+//  V8_HAS_BUILTIN_CTZ                  - __builtin_ctz() supported
 //  V8_HAS_BUILTIN_EXPECT               - __builtin_expect() supported
+//  V8_HAS_BUILTIN_POPCOUNT             - __builtin_popcount() supported
 //  V8_HAS_DECLSPEC_ALIGN               - __declspec(align(n)) supported
 //  V8_HAS_DECLSPEC_DEPRECATED          - __declspec(deprecated) supported
 //  V8_HAS_DECLSPEC_NOINLINE            - __declspec(noinline) supported
@@ -206,7 +209,10 @@
 # define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT \
     (__has_attribute(warn_unused_result))
 
+# define V8_HAS_BUILTIN_CLZ (__has_builtin(__builtin_clz))
+# define V8_HAS_BUILTIN_CTZ (__has_builtin(__builtin_ctz))
 # define V8_HAS_BUILTIN_EXPECT (__has_builtin(__builtin_expect))
+# define V8_HAS_BUILTIN_POPCOUNT (__has_builtin(__builtin_popcount))
 
 # define V8_HAS_CXX11_ALIGNAS (__has_feature(cxx_alignas))
 # define V8_HAS_CXX11_STATIC_ASSERT (__has_feature(cxx_static_assert))
@@ -238,7 +244,10 @@
 # define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT \
     (!V8_CC_INTEL && V8_GNUC_PREREQ(4, 1, 0))
 
+# define V8_HAS_BUILTIN_CLZ (V8_GNUC_PREREQ(3, 4, 0))
+# define V8_HAS_BUILTIN_CTZ (V8_GNUC_PREREQ(3, 4, 0))
 # define V8_HAS_BUILTIN_EXPECT (V8_GNUC_PREREQ(2, 96, 0))
+# define V8_HAS_BUILTIN_POPCOUNT (V8_GNUC_PREREQ(3, 4, 0))
 
 // g++ requires -std=c++0x or -std=gnu++0x to support C++11 functionality
 // without warnings (functionality used by the macros below).  These modes