XL: Use -qpic for position independent executables

The XL compiler does not have a dedicated "pie" option so use at least
`-qpic`.  This missing setting was exposed by the `ExportImport` and
`PositionIndependentTargets` tests with XL on an ELF platform.

Modules/Compiler/XL.cmake

@@ -20,6 +20,7 @@ macro(__compiler_xl lang)
   # Feature flags.
   set(CMAKE_${lang}_VERBOSE_FLAG "-V")
   set(CMAKE_${lang}_COMPILE_OPTIONS_PIC "-qpic")
+  set(CMAKE_${lang}_COMPILE_OPTIONS_PIE "-qpic")
   set(CMAKE_${lang}_RESPONSE_FILE_FLAG "-qoptfile=")
   set(CMAKE_${lang}_RESPONSE_FILE_LINK_FLAG "-qoptfile=")