[android] build libxml2 and curl with -fPIC

Sets `CMAKE_POSITION_INDEPENDENT_CODE=YES` when building libxml2 and
curl. This causes `-fPIC` to be passed to clang when compiling, which is
required for the i686 Android build to succeed.

This change makes `build.ps1` consistent with the Python build files
swift_build_support\products\libxml2.py and
swift_build_support\products\curl.py, which already define this cmake
property. It is already propertly set for zlib in both locations.

Author: andrurogerz

utils/build.ps1

@@ -1502,6 +1502,7 @@ function Build-XML2([Platform]$Platform, $Arch) {
       BUILD_SHARED_LIBS = "NO";
       CMAKE_INSTALL_BINDIR = "bin/$Platform/$ArchName";
       CMAKE_INSTALL_LIBDIR = "lib/$Platform/$ArchName";
+      CMAKE_POSITION_INDEPENDENT_CODE = "YES";
       CMAKE_SYSTEM_NAME = $Platform.ToString();
       LIBXML2_WITH_ICONV = "NO";
       LIBXML2_WITH_ICU = "NO";
@@ -1568,6 +1569,7 @@ function Build-CURL([Platform]$Platform, $Arch) {
       BUILD_SHARED_LIBS = "NO";
       BUILD_TESTING = "NO";
       CMAKE_INSTALL_LIBDIR = "lib/$Platform/$ArchName";
+      CMAKE_POSITION_INDEPENDENT_CODE = "YES";
       CMAKE_SYSTEM_NAME = $Platform.ToString();
       BUILD_CURL_EXE = "NO";
       BUILD_LIBCURL_DOCS = "NO";