[PATCH 0/5] Improvements with __attribute__((packed))

[Andrew Cooper <andrew.cooper3@citrix.com>]

Due to a lack of suitable define in scope, mce-apei.c ends up accidentally
creating a global unreferenced struct of type 'cper_mce_record', named
'__packed'.

This series is a cleanup of all use of __attribute__((packed)) in Xen.  A
formal define is created in compiler.h, all opencoded uses of the attribute
are updated to use the new define (or dropped if unnecessary in the first
place), and the position of __packed is standardised at the beginning of the
struct rather than the end, so a lack of __packed in scope will result in a
compile error.

Patch 1 drops some redundant uses of __attribute__((packed)), and is verified
by diffing xen-syms.

Patches 2 through 4 make the main changes, and are split up by core component
for the benefit of the maintainers/reviewers.

Patch 5 is only for people wishing to verify my assertions.  The entire
series including patch 5 compiles to identical binaries, proving that the
reverse contents of patch 5 are the only semantic differences across the
series.

This is compile tested on each architecture and functionally tested on x86.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>