[CMake] Handle 'Generic' system in HandleLLVMOptions.cmake

[JamesNagurne]

Summary

This configuration file is included in libcxx/libcxxabi/libunwind build
systems, which should be able to be built for embedded ARM, which is
considered a 'Generic' system (CMAKE_SYSTEM_NAME).

However, this file errors out when neither WIN32, UNIX, nor FUCHSIA is set.

I've defined the generic system as being neither WIN32, UNIX, and doesn't
have the shared library support for a link version script. Looking through
the applications of the variables defined, I couldn't see any conflicts
in the aforementioned projects.

[phosek]

LGTM

llvm/cmake/modules/HandleLLVMOptions.cmake

if(WIN32)
   set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
   if(CYGWIN)
     set(LLVM_ON_WIN32 0)
     set(LLVM_ON_UNIX 1)
   else(CYGWIN)
     set(LLVM_ON_WIN32 1)
     set(LLVM_ON_UNIX 0)
   endif(CYGWIN)
-else(WIN32)
-  if(FUCHSIA OR UNIX)
-    set(LLVM_ON_WIN32 0)
-    set(LLVM_ON_UNIX 1)
-    if(APPLE OR ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
-      set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
-    else()
-      set(LLVM_HAVE_LINK_VERSION_SCRIPT 1)
-    endif()
-  else(FUCHSIA OR UNIX)
-    MESSAGE(SEND_ERROR "Unable to determine platform")
-  endif(FUCHSIA OR UNIX)
-endif(WIN32)
+elseif(FUCHSIA OR UNIX)
+elseif(CMAKE_SYSTEM_NAME STREQUAL "Generic")
+  set(LLVM_ON_WIN32 0)
+  set(LLVM_ON_UNIX 0)
+  set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
+else()
+endif()
 
 set(EXEEXT ${CMAKE_EXECUTABLE_SUFFIX})
 set(LTDL_SHLIB_EXT ${CMAKE_SHARED_LIBRARY_SUFFIX})
 
 # We use *.dylib rather than *.so on darwin.
 set(LLVM_PLUGIN_EXT ${CMAKE_SHARED_LIBRARY_SUFFIX})
 
 if(APPLE)