[build-script] When building with Xcode, do not build or test any stdlib but the macosx-x86_64 stdlib.

There is a known bug where Xcode can not properly cross compile swift standard
libraries since it always wants to pass in -mmacosx-version-min (see
CMakeLists.txt:512).

Even though this code is in CMake, build-script if it is not told to do the
correct thing, will still try to build those cross compiled libraries. This will
fail since cmake did not create a target.

Author: gottesmm

utils/build-presets.ini

@@ -249,10 +249,21 @@ skip-test-watchos
 
 [preset: buildbot_incremental,tools=RA,stdlib=RA,xcode]
 mixin-preset=buildbot_incremental,tools=RA,stdlib=RA
-build-subdir=buildbot_incremental-xcode
+build-subdir=buildbot_incremental_xcode
 
 xcode
 dash-dash
+# We do not support building cross compiled stdlibs on OS X with Xcode. So only
+# build the OS X SDK.
+skip-build-ios
+skip-test-ios
+skip-build-tvos
+skip-test-tvos
+skip-build-watchos
+skip-test-watchos
+stdlib-deployment-targets=macosx-x86_64
+swift-primary-variant-sdk=OSX
+swift-primary-variant-arch=x86_64
 
 # This preset is used by CI to test swift-corelibs-xctest.
 [preset: buildbot_incremental,tools=RA,stdlib=RA,XCTest]