Revert to shell execution when invoking nm tool

This reverts a change of commit b3598cf .
On Windows on ARM64 with LLVM the "NM" tool is called with a parameter like so:
```
  RbConfig::CONFIG["NM"] # => "llvm-nm --no-llvm-bc"
```

Therefore the command must be called with a shell string.

Author: larskanis

win32/mkexports.rb

@@ -146,9 +146,7 @@ def exports(*)
   end
 
   def each_line(objs, &block)
-    IO.popen(%W[#{self.class.nm} --extern-only --defined-only] + objs) do |f|
-      f.each(&block)
-    end
+    IO.foreach("|#{self.class.nm} --extern-only --defined-only #{objs.join(' ')}", &block)
   end
 
   def each_export(objs)