Fix XL C++ compiler flags on Linux

In Platform/Linux.cmake we add GNU flags as default for the platform
which breaks non-GNU compilers.  Later we should refactor these flag
files to put compiler-specific flags only in files loaded for each
compiler.  Until then this commit fixes the XL C++ compiler flags on
Linux by erasing the GNU flags.  See issue #9469.

Modules/Platform/Linux-VisualAge-CXX.cmake

@@ -0,0 +1 @@
+include(Platform/Linux-XL-CXX)

Modules/Platform/Linux-XL-CXX.cmake

@@ -0,0 +1,5 @@
+SET(CMAKE_SHARED_LIBRARY_C_FLAGS "")
+SET(CMAKE_SHARED_LIBRARY_LINK_C_FLAGS "")
+SET(CMAKE_SHARED_LIBRARY_CXX_FLAGS "")
+SET(CMAKE_SHARED_LIBRARY_LINK_CXX_FLAGS "")
+SET(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-qmkshrobj")