Adjust how gcc4 handles the "missing braces around initializer" warning.

Adjust how gcc4 handles the "missing braces around initializer" warning.

In c99 any value can be initalised using a { 0 } constructor independent
of the type. Now if a struct's first member is another struct then gcc4
issues the above warning but it should not do that.

Move the warning check from push_init_level() to pop_init_level() and
check if either { 0 } or { } was used. If additional implicit braces
were added surpress the warning.

Inspired by gcc PR#64709
OK deraadt@ miod@