[gdb/testsuite] Use c++ flag in c++ test-cases

In some cases, test-cases use c++, but don't add "c++" to the compilation
flags.  This can cause problems with some compilers.

Fix this in some test-cases.

Approved-By: Tom Tromey <tom@tromey.com>

PR testsuite/30380
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30380