Allow building with CMAKE_SYSTEM_NAME=Generic

This is important for building runtimes for bare metal targets.

Differential Revision: https://reviews.llvm.org/D144757

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -141,19 +141,21 @@ if(WIN32)
     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)
+  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()
+elseif(CMAKE_SYSTEM_NAME STREQUAL "Generic")
+  set(LLVM_ON_WIN32 0)
+  set(LLVM_ON_UNIX 0)
+  set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
+else()
+  MESSAGE(SEND_ERROR "Unable to determine platform")
+endif()
 
 if (CMAKE_SYSTEM_NAME MATCHES "OS390")
   set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)