bsd-user/x86_64/target_arch_thread.h: Align stack

bsd-user/x86_64/target_arch_thread.h: Align stack

bsd-user qemu-x86_64 almost immediately dies with:

    qemu: 0x4002201a68: unhandled CPU exception 0xd - aborting

on FreeBSD 14.1-RELEASE. This is an instruction that requires
alignment:

    (gdb) x/i 0x4002201a68
       0x4002201a68:        movaps %xmm0,-0x40(%rbp)

and the argument is not aligned:

    (gdb) p/x env->regs[5]
    $1 = 0x822443b58

A quick experiment shows that the userspace entry point expects
misaligned rsp:

    (gdb) starti
    (gdb) p/x $rsp
    $1 = 0x7fffffffeaa8

Emulate this behavior in bsd-user.

[[ applied Richard's suggestion ]]

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Warner Losh <imp@bsdimp.com>