Enable -Wshadow and fix resulting errors

Prompted by https://github.com/Linaro/vixl/pull/9 enabled -Wshadow generally.
diff --git a/SConstruct b/SConstruct
index cc15825..bb8638c 100644
--- a/SConstruct
+++ b/SConstruct
@@ -84,6 +84,7 @@
                    '-pedantic',
                    '-Wwrite-strings',
                    '-Wunused',
+                   '-Wshadow',
                    '-Wno-missing-noreturn'],
       'CPPPATH' : [config.dir_src_vixl]
       },