[JITLink][aarch64] Pointer jump stubs should have 4-byte alignment.

Pointer jump stubs content was incorrectly using 1-byte alignment. This bug
flew under the radar because the ordinary instruction streams (and aligned
section starts) usually left the block 4-byte aligned anyway. The bug was
observed when an out-of-tree pass introduced new content with arbitrary sizes
into the text section.

No testcase yet: We don't have a good way to replicate the perturbation that
was caused by the out-of-tree pass.