Enable -Wextra-semi on compilers that support it

Use some template magic to make warning detection much easier in the future.
diff --git a/build.yaml b/build.yaml
index 5e47c08..65e80a9 100644
--- a/build.yaml
+++ b/build.yaml
@@ -3252,7 +3252,7 @@
 defaults:
   boringssl:
     CFLAGS: -Wno-sign-conversion -Wno-conversion -Wno-unused-value -Wno-unknown-pragmas
-      -Wno-implicit-function-declaration -Wno-unused-variable -Wno-sign-compare
+      -Wno-implicit-function-declaration -Wno-unused-variable -Wno-sign-compare $(NO_W_EXTRA_SEMI)
     CPPFLAGS: -Ithird_party/boringssl/include -fvisibility=hidden -DOPENSSL_NO_ASM
       -D_GNU_SOURCE -DWIN32_LEAN_AND_MEAN -D_HAS_EXCEPTIONS=0 -DNOMINMAX
   global: