add missing lower() to fix msvc backend platform detection

Regardless of which MachineChoice we base the platform on, we compare
its value to lowercased identifiers. So we need to lowercase the
targetplatform too... but we only did so sometimes.

This broke e.g. on "Win32", but only when *not* doing a cross build.

Fixes #10539

mesonbuild/backend/vs2010backend.py

@@ -1312,7 +1312,7 @@ class Vs2010Backend(backends.Backend):
             pdb.text = f'$(OutDir){target_name}.pdb'
         targetmachine = ET.SubElement(link, 'TargetMachine')
         if target.for_machine is MachineChoice.BUILD:
-            targetplatform = platform
+            targetplatform = platform.lower()
         else:
             targetplatform = self.platform.lower()
         if targetplatform == 'win32':