[gdb/testsuite] Require c++11 for gdb.base/align.exp

[gdb/testsuite] Require c++11 for gdb.base/align.exp

When building gdb on ubuntu 16.04 with gcc 5.4.0, and running the gdb
testsuite we run into a failure due align.exp requiring at least c++11.

Fix this by adding -std=c++11.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-05-22  Tom de Vries  <tdevries@suse.de>

	* gdb.base/align.exp: Require c++11.