[Object] Fix handling of Elf_Nhdr with sh_addralign=8

The generic ABI says:

> Padding is present, if necessary, to ensure 8 or 4-byte alignment for the next note entry (depending on whether the file is a 64-bit or 32-bit object). Such padding is not included in descsz.

Our parsing code currently aligns n_namesz. Fix the bug by aligning the start
offset of the descriptor instead. This issue has been benign because the primary
uses of sh_addralign=8 notes are `.note.gnu.property`, where
`sizeof(Elf_Nhdr) + sizeof("GNU") = 16` (already aligned by 8).

In practice, many 64-bit systems incorrectly use sh_addralign=4 notes.
We can use sh_addralign (= p_align) to decide the descriptor padding.
Treat an alignment of 0 and 1 as 4. This approach matches modern GNU readelf
(since 2018).

We have a few tests incorrectly using sh_addralign=0. We may make our behavior
stricter after fixing these tests.

Linux kernel dumped core files use `p_align=0` notes, so we need to support the
case for compatibility.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D150022