Respect SWIFT_TOOLCHAIN_MACOS_DEPLOYMENT_TARGET when set (such as during Swift toolchain build)

If it is not set in the environment, stay with the default minimum deployment target of 10.10 on macOS.

This aligns with both swift-package-manager and with swift-driver, and is important now that ToolSupportCore has started containing some code that is annotated with availability for Darwin platforms.  It uses the more project-neutral name `SWIFT_TOOLCHAIN_MACOS_DEPLOYMENT_TARGET` and the expectation is that both SwiftPM and SwiftDriver will switch to using it as well.

Author: abertelrud

Package.swift

@@ -3,7 +3,7 @@
 /*
  This source file is part of the Swift.org open source project
 
- Copyright (c) 2019 - 2020 Apple Inc. and the Swift project authors
+ Copyright (c) 2019 - 2021 Apple Inc. and the Swift project authors
  Licensed under Apache License v2.0 with Runtime Library Exception
 
  See http://swift.org/LICENSE.txt for license information
@@ -12,9 +12,20 @@
 
 
 import PackageDescription
+import class Foundation.ProcessInfo
+
+let macOSPlatform: SupportedPlatform
+if let deploymentTarget = ProcessInfo.processInfo.environment["SWIFT_TOOLCHAIN_MACOS_DEPLOYMENT_TARGET"] {
+    macOSPlatform = .macOS(deploymentTarget)
+} else {
+    macOSPlatform = .macOS(.v10_10)
+}
 
 let package = Package(
     name: "swift-tools-support-core",
+    platforms: [
+        macOSPlatform,
+    ],
     products: [
         .library(
             name: "SwiftToolsSupport",