[gdb/testsuite] Handle some test-cases with older compiler

When running test-case gdb.mi/print-simple-values.exp with gcc 4.8.4, I run
into a compilation failure due to the test-case requiring c++11 and the
compiler defaulting to less than that.

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

Likewise in a few other test-cases.

Tested on x86_64-linux.