If MSVC and cross compiling use CMAKE_NM for gen_exports_msvc

[lanza]

Summary

LLVM-C.dll was recently set to build by default. However, the script
attempts to use the just-built llvm-nm. If you are crosscompiling then
this tool is for the wrong platform. In the case that you are
croscompiling, instead opt to use the CMAKE_NM.

[smeenai]

Sorry, I went down a rabbit hole reviewing this, and ended up writing an alternate solution which just generates a native llvm-nm using the same mechanism as tblgen. Gimme a little bit to put that up.

[lanza]

@smeenai sounds good to me!

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 we are crosscompiling with clang-cl MSVC reports as true yet the llvm-nm
+  # in the build directory can not run on the host. If so, default to the nm
+  # found by cmake.
+  if (CMAKE_CROSSCOMPILING)
+    if (CMAKE_NM)
+      set(GEN_NM ${CMAKE_NM})
+    else()
+      message(FATAL_ERROR "Building LLVM-C requires the nm tool. Either disable it or set -DCMAKE_NM=/path/to/nm")
+    endif()
+  else()
+    set(GEN_NM ${LLVM_TOOLS_BINARY_DIR}/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}
+    COMMAND ${PYTHON_EXECUTABLE} ${GEN_SCRIPT} --libsfile ${LIBSFILE} ${GEN_UNDERSCORE} --nm ${GEN_NM} -o ${LLVM_EXPORTED_SYMBOL_FILE}
     DEPENDS ${LIB_NAMES} llvm-nm
     COMMENT "Generating export list for LLVM-C"
     VERBATIM )
 
   # Finally link the target.
   add_llvm_library(LLVM-C SHARED ${SOURCES} DEPENDS intrinsics_gen)
 
 endif()