XL: Use -qpic for position independent code (#14010)

According to XL C/C++ V9.0 documentation the default for -qpic/-qnopic
is platform-dependent.  It won't hurt to add the option on platforms
where it is the default, so always add it when we want position
independent code.

Modules/Compiler/XL.cmake

@@ -27,6 +27,7 @@ find_program(CMAKE_XL_CreateExportList
 macro(__compiler_xl lang)
   # Feature flags.
   set(CMAKE_${lang}_VERBOSE_FLAG "-V")
+  set(CMAKE_${lang}_COMPILE_OPTIONS_PIC "-qpic")
 
   set(CMAKE_${lang}_FLAGS_DEBUG_INIT "-g")
   set(CMAKE_${lang}_FLAGS_RELEASE_INIT "-O")