[TestBitfields] Correct the attribute name to "aligned" in the testcase.

Summary:
This makes all parts of the test pass with Clang and GCC. They are
enabled with this patch.

Test Plan: dotest.py -C <clang|gcc> -p TestBitfields

Reviewers: vharron, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8874

llvm-svn: 234523