[cleanup] extern "C" not needed for V8_Fatal

Why I want to fix this: I got a CL to replace V8_NORETURN by
[[noreturn]], but clang-format formats this as
  extern "C"[[noreturn]] PRINT_FORMAT...
(i.e. missing whitespace).
Also, this is the only extern "C" function in our code base, so if we
do not need to call it from C, we should just get rid of it.

R=jochen@chromium.org
BUG=v8:6474

Change-Id: I950bdc505822eb37a107c58e63c82a61907ba515
Reviewed-on: https://chromium-review.googlesource.com/539341Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46149}

src/base/logging.cc

@@ -109,7 +109,7 @@ DEFINE_CHECK_OP_IMPL(GT)
 
 
 // Contains protection against recursive calls (faults while handling faults).
-extern "C" void V8_Fatal(const char* file, int line, const char* format, ...) {
+void V8_Fatal(const char* file, int line, const char* format, ...) {
   fflush(stdout);
   fflush(stderr);
   v8::base::OS::PrintError("\n\n#\n# Fatal error in %s, line %d\n# ", file,

src/base/logging.h

@@ -13,7 +13,7 @@
 #include "src/base/build_config.h"
 #include "src/base/compiler-specific.h"
 
-extern "C" V8_NORETURN PRINTF_FORMAT(3, 4) V8_BASE_EXPORT
+V8_NORETURN PRINTF_FORMAT(3, 4) V8_BASE_EXPORT
     void V8_Fatal(const char* file, int line, const char* format, ...);
 
 // The FATAL, UNREACHABLE and UNIMPLEMENTED macros are useful during