[gdb/testsuite] Fix gdb.cp/align.exp with gcc 12.1 / 11.3

[gdb/testsuite] Fix gdb.cp/align.exp with gcc 12.1 / 11.3

Starting with gcc 12.1 / gcc 11.3, for test-case gdb.cp/align.exp we run into:
...
align.cc:29:23: error: invalid application of 'alignof' to a void type^M
   29 |     unsigned a_void = alignof (void);^M
      |                       ^~~~~~~~~~~~~~^M
...

Fix this by using __alignof__ instead.

Tested on x86_64-linux, with gcc 7.5.0, gcc 12.1 and clang 12.0.1.