[llvm-shlib] Fix cross-compilation for LLVM-C

[smeenai]

Summary

When we're cross-compiling, build and use a native llvm-nm instead of
attempting to use the one from the target's build tree.

A nice follow-up would be to add a cache variable to allow specifying a
path to an external native llvm-nm instead of building one ourselves,
similar to LLVM_TABLEGEN and LLVM_CONFIG_PATH.

[phosek]

LGTM

[smeenai]

Rebase

[Wallbraker]

I can't comment to much on the cmake as my cmake-fu is weak, but from what I can tell it looks good.

llvm/tools/llvm-shlib/CMakeLists.txt

else()
     file(WRITE ${LIBSFILE} "${FILE_CONTENT}")
   endif()
 
   # Generate the exports file dynamically.
   set(GEN_SCRIPT ${CMAKE_CURRENT_SOURCE_DIR}/gen-msvc-exports.py)
 
   set(LLVM_EXPORTED_SYMBOL_FILE ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/libllvm-c.exports)
 
+  if(CMAKE_CROSSCOMPILING)
+    build_native_tool(TARGET llvm-nm
+                      OUTPUT_PATH_VAR llvm_nm)
+    set(llvm_nm_target "${llvm_nm}")
+  else()
+    set(llvm_nm $<TARGET_FILE:llvm-nm>)
+    set(llvm_nm_target llvm-nm)
+  endif()
+
   add_custom_command(OUTPUT ${LLVM_EXPORTED_SYMBOL_FILE}
-    COMMAND ${PYTHON_EXECUTABLE} ${GEN_SCRIPT} --libsfile ${LIBSFILE} ${GEN_UNDERSCORE} --nm ${LLVM_TOOLS_BINARY_DIR}/llvm-nm -o ${LLVM_EXPORTED_SYMBOL_FILE}
-    DEPENDS ${LIB_NAMES} llvm-nm
+    COMMAND ${PYTHON_EXECUTABLE} ${GEN_SCRIPT} --libsfile ${LIBSFILE} ${GEN_UNDERSCORE} --nm "${llvm_nm}" -o ${LLVM_EXPORTED_SYMBOL_FILE}
+    DEPENDS ${LIB_NAMES} ${llvm_nm_target}
     COMMENT "Generating export list for LLVM-C"
     VERBATIM )
 
   # Finally link the target.
   add_llvm_library(LLVM-C SHARED ${SOURCES} DEPENDS intrinsics_gen)
 
 endif()