Fix gdb.base/align-*.exp and Clang + LTO and AIX GCC

Clang with LTO (clang -flto) garbage collects unused global variables,
Thus, gdb.base/align-c.exp and gdb.base/align-c++.exp fail with
hundreds of FAILs like so:

 $ make check \
    TESTS="gdb.*/align-*.exp" \
    RUNTESTFLAGS="CC_FOR_TARGET='clang -flto' CXX_FOR_TARGET='clang++ -flto'"
 ...
 FAIL: gdb.base/align-c.exp: get integer valueof "a_char"
 FAIL: gdb.base/align-c.exp: print _Alignof(char)
 FAIL: gdb.base/align-c.exp: get integer valueof "a_char_x_char"
 FAIL: gdb.base/align-c.exp: print _Alignof(struct align_pair_char_x_char)
 FAIL: gdb.base/align-c.exp: get integer valueof "a_char_x_unsigned_char"
 ...

AIX GCC has the same issue, and there the easier way of adding
__attribute__((used)) to globals does not help.

So add explicit uses of all globals to the generated code.

For the C++ test, that reveals that the static variable members of the
generated structs are not defined anywhere, leading to undefined
references.  Fixed by emitting initialization for all static members.

Lastly, I noticed that CXX_FOR_TARGET was being ignored -- that's
because the align-c++.exp testcase is compiling with the C compiler
driver.  Fixed by passing "c++" as option to prepare_for_testing.

Change-Id: I874b717afde7b6fb1e45e526912b518a20a12716