Respect CMAKE_SYSROOT and don't search out libxml2 if it is set.

[srhines]

Summary

If we ignore CMAKE_SYSROOT here, we end up always using the host
information for libxml2, when it is not appropriate. This makes builds
less hermetic than they should ideally be.

[srhines]

Removed this in favor of the suggestions in https://reviews.llvm.org/D48459. Setting the CMAKE_FIND_ROOT_PATH_MODE* variables does make this properly hermetic.

cmake/config-ix.cmake

if(LLVM_ENABLE_TERMINFO)
       endforeach()
     else()
       set(HAVE_TERMINFO 0)
     endif()
 
     find_library(ICONV_LIBRARY_PATH NAMES iconv libiconv libiconv-2 c)
     set(LLVM_LIBXML2_ENABLED 0)
     set(LIBXML2_FOUND 0)
-    if((LLVM_ENABLE_LIBXML2) AND ((CMAKE_SYSTEM_NAME MATCHES "Linux") AND (ICONV_LIBRARY_PATH) OR APPLE))
+    if((LLVM_ENABLE_LIBXML2) AND ((CMAKE_SYSTEM_NAME MATCHES "Linux") AND (ICONV_LIBRARY_PATH) OR APPLE) AND NOT CMAKE_SYSROOT)
       find_package(LibXml2)
       if (LIBXML2_FOUND)
         set(LLVM_LIBXML2_ENABLED 1)
         if ((CMAKE_OSX_SYSROOT) AND (EXISTS ${CMAKE_OSX_SYSROOT}/${LIBXML2_INCLUDE_DIR}))
           include_directories(${CMAKE_OSX_SYSROOT}/${LIBXML2_INCLUDE_DIR})
         else()
           include_directories(${LIBXML2_INCLUDE_DIR})
         endif()