Add option LLVM_NM to allow specifying the location of the llvm-nm tool

The new option works like the existing LLVM_TABLEGEN, and
LLVM_CONFIG_PATH options.  Instead of building llvm-nm, the build uses
the executable defined by LLVM_NM.

This is useful for cross-compilation scenarios where the host cannot run
the cross-compiled tool, and recursing into another cmake build is not
an option (due to required DEFINE's, for example).

Reviewed By: smeenai

Differential Revision: https://reviews.llvm.org/D83022

llvm/tools/llvm-shlib/CMakeLists.txt

@@ -154,7 +154,7 @@ if(LLVM_BUILD_LLVM_C_DYLIB AND MSVC)
   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(NOT LLVM_NM)
     if(CMAKE_CROSSCOMPILING)
       build_native_tool(llvm-nm llvm_nm)
       set(llvm_nm_target "${llvm_nm}")
@@ -162,6 +162,10 @@ if(LLVM_BUILD_LLVM_C_DYLIB AND MSVC)
       set(llvm_nm $<TARGET_FILE:llvm-nm>)
       set(llvm_nm_target llvm-nm)
     endif()
+  else()
+    set(llvm_nm ${LLVM_NM})
+    set(llvm_nm_target "")
+  endif()
 
   add_custom_command(OUTPUT ${LLVM_EXPORTED_SYMBOL_FILE}
     COMMAND "${Python3_EXECUTABLE}" ${GEN_SCRIPT} --libsfile ${LIBSFILE} ${GEN_UNDERSCORE} --nm "${llvm_nm}" -o ${LLVM_EXPORTED_SYMBOL_FILE}