Fix stat decoding for LP64 bionic

Patch fb642bb6d63f7ffe2228bf48a6008bc8f56f67ff fixed building with
HAVE_STAT64 for aarch64 with uapi kernel headers but not x86_64.
The workaround needed to be applied to all LP64 architectures, not
just aarch64.  This patch fixes that and adds an explanatory comment.

Signed-off-by: Elliott Hughes <enh@google.com>