[build.ps1] add host SDK path when building foundation for Android on windows

This lets us build the FoundationMacros for the windows host

Author: hyp

utils/build.ps1

@@ -1640,6 +1640,11 @@ function Build-Foundation([Platform]$Platform, $Arch, [switch]$Test = $false) {
       $Targets = @("default", "install")
       $InstallPath = "$($Arch.SDKInstallRoot)\usr"
     }
+    if ($Platform -eq "Android") {
+      $HostDefines = @{ CMAKE_HOST_Swift_FLAGS = "-sdk `"$($HostArch.SDKInstallRoot)`"" }
+    } else {
+      $HostDefines = @{}
+    }
 
     $env:CTEST_OUTPUT_ON_FAILURE = 1
     Build-CMakeProject `
@@ -1671,7 +1676,7 @@ function Build-Foundation([Platform]$Platform, $Arch, [switch]$Test = $false) {
         _SwiftFoundation_SourceDIR = "$SourceCache\swift-foundation";
         _SwiftFoundationICU_SourceDIR = "$SourceCache\swift-foundation-icu";
         _SwiftCollections_SourceDIR = "$SourceCache\swift-collections"
-      } + $TestingDefines)
+      } + $HostDefines + $TestingDefines)
   }
 }