[platform] Fix compilation on 32-bit Windows

Use `extern "C"` to declare the __readfsdword function.
Fixes error C2732 in the Node.js CI.

Bug: chromium:796644
Change-Id: If261985e65bfdade53ce06ff28afe0e2db402f7a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3086457Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Michaël Zasso <mic.besace@gmail.com>
Cr-Commit-Position: refs/heads/master@{#76259}

src/base/platform/platform.h

@@ -47,7 +47,7 @@
 // And, intrin.h is a very expensive header that we want to avoid here, and
 // the cheaper intrin0.h is not available for all build configurations. That is
 // why we declare this intrinsic.
-unsigned long __readfsdword(unsigned long);  // NOLINT(runtime/int)
+extern "C" unsigned long __readfsdword(unsigned long);  // NOLINT(runtime/int)
 #endif                                       // V8_CC_MSVC && V8_HOST_ARCH_IA32
 #endif                                       // V8_NO_FAST_TLS