BinUtils: Use more-private temporary variable names

Since commit 4d786dfcfa (BinUtils: Avoid clobbering a variable named
without a private prefix, 2021-04-06, v3.20.1~4^2) we use variables
named `_tool` and `_name`, but these may still be common enough to
affect project code.  Use `_CMAKE_TOOL` and `_CMAKE_TOOL_NAME` instead,
and unset them when finished.

Fixes: #22140