[fuchsia] Enable shadow-call-stack for all targets.

Bug: fuchsia:27339, 821951
Change-Id: If685fd9817f8dae8671634f3798a655ddb2ba392
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863774
Auto-Submit: Wez <wez@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709978}

build/config/fuchsia/BUILD.gn

@@ -51,7 +51,11 @@ config("compiler") {
 
   # TODO(crbug.com/821951): Clang enables SafeStack by default when targeting
   # Fuchsia, but it breaks some tests, notably in V8.
-  cflags += [ "-fno-sanitize=safe-stack" ]
+  # Force the toolchain to use shadow-call-stack instead, until that is default.
+  cflags += [
+    "-fno-sanitize=safe-stack",
+    "-fsanitize=shadow-call-stack",
+  ]
 
   libs = [ "zircon" ]
 }