[libc++] Normalize all our '#pragma GCC system_header', and regression-test.

Now we'll notice if a header forgets to include this magic phrase.

Differential Revision: https://reviews.llvm.org/D118800

NOKEYCHECK=True
GitOrigin-RevId: fa6b9e401085c0a210daf81ae79af6dd0144f129
diff --git a/include/stdexcept b/include/stdexcept
index 5df2050..f6dcf5f 100644
--- a/include/stdexcept
+++ b/include/stdexcept
@@ -47,7 +47,7 @@
 #include <iosfwd>  // for string forward decl
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
+#  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD