x86: Align L2's stacks

Setting the stack to PAGE_SIZE - 1 sets the stack to being 1-byte
aligned, which fails in usermode with alignment checks enabled (ie: with
flags cr0.am set and eflags.ac set).  This was causing an #AC in
usermode.c when preparing to call the callback in run_in_user().
Aligning the stack fixes the issue.

For the purposes of fixing the #AC in usermode.c the stack has to be
aligned to at least an 8-byte boundary.  Setting it to a page aligned
boundary ensures any stack alignment requirements are met as x86_64
stacks generally want to be 16-byte aligned.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Message-Id: <20211214011823.3277011-3-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>