[Apple Silicon] Fix cross-compilation in Xcode trains where we're not building stdlib at all

Author: shahmishal

utils/build-script-impl

@@ -1056,6 +1056,10 @@ function host_has_darwin_symbols() {
 }
 
 function get_stdlib_targets_for_host() {
+    # Don't build the stdlib in the Xcode train for host and cross-compilations host.
+    if [[ "${STDLIB_DEPLOYMENT_TARGETS[@]}" == "" ]]; then
+        return 0
+    fi
 
 # FIXME: STDLIB_DEPLOYMENT_TARGETS argument assumed to apply when Host == Build
 # Cross-compile Hosts are only built with their native standard libraries.

utils/swift_build_support/swift_build_support/host_specific_configuration.py

@@ -41,6 +41,10 @@ def __init__(self, host_target, args):
             stdlib_targets_to_configure = [host_target]
             stdlib_targets_to_build = set(stdlib_targets_to_configure)
 
+        if args.stdlib_deployment_targets == []:
+            stdlib_targets_to_configure = []
+            stdlib_targets_to_build = []
+
         # Compute derived information from the arguments.
         #
         # FIXME: We should move the platform-derived arguments to be entirely