replace prep_libc.sh with prep_libc.py, and quiet the output

base/allocator/allocator.gyp

Index: base/allocator/allocator.gyp
diff --git a/base/allocator/allocator.gyp b/base/allocator/allocator.gyp
index 7b1058c10a96a4e15e09c45818ee85d4991ac12c..1eccb442e4eec9ea51d5fb8d9dc752f04f5e2abe 100644
--- a/base/allocator/allocator.gyp
+++ b/base/allocator/allocator.gyp
@@ -453,16 +453,18 @@
             {
               'action_name': 'libcmt',
               'inputs': [
-                'prep_libc.sh',
+                'prep_libc.py',
               ],
               'outputs': [
                 '<(SHARED_INTERMEDIATE_DIR)/allocator/libcmt.lib',
               ],
               'action': [
-                './prep_libc.sh',
+                'python',
+                'prep_libc.py',
                 '$(VCInstallDir)lib',
                 '<(SHARED_INTERMEDIATE_DIR)/allocator',
               ],
+              'msvs_cygwin_shell': '0',
             },
           ],
         },