[gdb/testsuite] Handle gdb.cp/*.exp with older compiler

[gdb/testsuite] Handle gdb.cp/*.exp with older compiler

When running test-cases gdb.cp/*.exp with gcc 4.8.4, I run into compilation
failures due to the test-cases requiring c++11 and the compiler defaulting
to less than that.

Fix this by compiling with -std=c++11.

This exposes two FAILs in gdb/testsuite/gdb.cp/empty-enum.exp due to
gcc PR debug/16063, so xfail those.

Also require have_compile_flag -std=c++17 in gdb.cp/constexpr-field.exp to
prevent compilation failure.

Tested on x86_64-linux.