Avoid "gcc -Wmissing-field-initializers" warnings on the generated code.

* src/options.h (Options::has_initializer_suffix): New declaration.
* src/options.cc (Options::has_initializer_suffix): New function.
* src/output.cc (Output::output_keyword_table): If option -t is
specified and option -F is not specified, emit '#pragma GCC diagnostic'
lines, to silence -Wmissing-field-initializers warnings from gcc or
clang.
* tests/charsets.exp: Update.
* tests/gpc.exp: Likewise.
* tests/incomplete.exp: Likewise.
* tests/languages.exp: Likewise.
* tests/objc.exp: Likewise.
* tests/test-4.exp: Likewise.