x86: reduce general stack alignment to 8

x86: reduce general stack alignment to 8

We don't need bigger alignment except when calling EFI boot or runtime
services functions (and we don't guarantee that either, as explained
close to the top of xen/common/efi/runtime.c in the struct efi_rs_state
declaration). Hence if the compiler supports reducing stack alignment
from the ABI compatible 16 bytes (gcc 7 and newer), do so wherever
possible.

The EFI case itself is largely dealt with already (actually forcing
32-byte alignment) as a result of commit f6b7fedc89 ("x86/EFI: meet
further spec requirements for runtime calls"). However, as explained in
the description of that earlier change, without using
-mincoming-stack-boundary=3 (which we don't want) we still have to make
the compiler assume 16-byte stack boundaries for CUs making EFI calls in
order to keep the compiler from aligning the stack, but then placing an
odd number of 8-byte objects on it, resulting in a mis-aligned outgoing
stack.

This as a side effect yields some code size reduction, since for a
number of sufficiently simple non-leaf functions the stack adjustment
(by 8, when there are no local stack variables at all) gets dropped
altogether. I notice exceptions though, for example in guest_cpuid(),
where in a release build gcc 8.2 now decides to set up a frame pointer
(without ever using %rbp); I consider this a compiler quirk which we
should leave to the compiler folks to address eventually.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>