[build] Dynamically decide if to use system xcode

This enables the recipe to set FORCE_MAC_TOOLCHAIN and decide if hermetic
or system xcode is used.

TBR=tmrts@chromium.org

Bug: chromium:986701
Change-Id: I2c1ba75d5d62e493bd9dc7a5ec5f7e40465d8f98
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1722557Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62947}

.gn

@@ -16,4 +16,5 @@ check_targets = []
 # These are the list of GN files that run exec_script. This whitelist exists
 # to force additional review for new uses of exec_script, which is strongly
 # discouraged except for gypi_to_gn calls.
-exec_script_whitelist = build_dotfile_settings.exec_script_whitelist + []
+exec_script_whitelist = build_dotfile_settings.exec_script_whitelist +
+                        [ "//build_overrides/build.gni" ]

build_overrides/build.gni

@@ -35,5 +35,16 @@ tsan_suppressions_file = "//build/sanitizers/tsan_suppressions.cc"
 # Skip assertions about 4GiB file size limit.
 ignore_elf32_limitations = true
 
-# Use the system install of Xcode for tools like ibtool, libtool, etc.
-use_system_xcode = true
+if (host_os == "mac") {
+  _result = exec_script("//build/mac/should_use_hermetic_xcode.py",
+                        [ target_os ],
+                        "value")
+  assert(_result != 2,
+         "Do not allow building targets with the default" +
+             "hermetic toolchain if the minimum OS version is not met.")
+  assert(_result != 3,
+         "iOS does not support building with a hermetic toolchain. " +
+             "Please install Xcode.")
+
+  use_system_xcode = _result == 0
+}