Add ASM platform information for XL compiler on AIX

Among other flags this sets RPATH flags correctly so that CMake knows
how to treat CMAKE_PLATFORM_REQUIRED_RUNTIME_PATH for the ASM language.

Modules/Platform/AIX-XL-ASM.cmake

@@ -0,0 +1,2 @@
+include(Platform/AIX-XL)
+__aix_compiler_xl(ASM)