Revert "[fuchsia] Stop overriding safestack & shadow-call-stack settings."

This reverts commit 4189e014.

Reason for revert: <may have broken blink_heap_unittests on fuschia>

Original change's description:
> [fuchsia] Stop overriding safestack & shadow-call-stack settings.
> 
> Fuchsia builds have been forcibly disabling safestack because it broke
> V8 tests under Fuchsia, and forcibly enabling shadow-call-stack in
> advance of it becoming the toolchain default, to gauge size impact.
> 
> Enabling shadow-call-stack under x64 is not recommended and breaks code
> which uses C++ exceptions, which is true of some V8 code & tests.
> 
> Since the toolchain default should now be correct, remove the overrides
> entirely.
> 
> Bug: 1028617, 821951
> Change-Id: I36333f8cff5ea62584032a802337cdeb95f33466
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014407
> Auto-Submit: Wez <wez@chromium.org>
> Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
> Commit-Queue: Wez <wez@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#734110}

TBR=wez@chromium.org,kmarshall@chromium.org

Change-Id: I182cb1fd966d08b9af68f5c50bb67928567cfb61
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1028617, 821951
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015355Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734165}

build/config/fuchsia/BUILD.gn

@@ -49,6 +49,14 @@ config("compiler") {
   # Add SDK lib dir for -lfdio above.
   lib_dirs = [ "${fuchsia_sdk}/arch/${current_cpu}/lib" ]
 
+  # TODO(crbug.com/821951): Clang enables SafeStack by default when targeting
+  # Fuchsia, but it breaks some tests, notably in V8.
+  # Force the toolchain to use shadow-call-stack instead, until that is default.
+  cflags += [
+    "-fno-sanitize=safe-stack",
+    "-fsanitize=shadow-call-stack",
+  ]
+
   libs = [ "zircon" ]
 }