[CMake] Support undefined LLVM_NATIVE_ARCH in llvm_map_components_to_libnames().

[kosarev]

Summary

https://reviews.llvm.org/D142405 made this function relying on the
LLVM_NATIVE_ARCH be defined, which is not necessarily the case for
third-party projects that include LLVM as their part.

[beanz]

LGTM

llvm/cmake/modules/LLVM-Config.cmake

if(engine IN_LIST link_components)
     if(${LLVM_NATIVE_ARCH} IN_LIST LLVM_TARGETS_TO_BUILD AND
        ${LLVM_NATIVE_ARCH} IN_LIST LLVM_TARGETS_WITH_JIT)
       list(APPEND link_components "jit")
       list(APPEND link_components "native")
     else()
       list(APPEND link_components "interpreter")
     endif()
   endif()
-  if(native IN_LIST link_components AND ${LLVM_NATIVE_ARCH} IN_LIST LLVM_TARGETS_TO_BUILD)
+  if(native IN_LIST link_components AND "${LLVM_NATIVE_ARCH}" IN_LIST LLVM_TARGETS_TO_BUILD)
     list(APPEND link_components ${LLVM_NATIVE_ARCH})
   endif()
 
   # Translate symbolic component names to real libraries:
   llvm_expand_pseudo_components(link_components ${link_components})
   foreach(c ${link_components})
     get_property(c_rename GLOBAL PROPERTY LLVM_COMPONENT_NAME_${c})
     if(c_rename)