x86: signal: use 16 bytes boundary for rt_sigframe

Impact: cleanup

Supporting xsave/xrestore introduces 64 bytes boundary for save_i387_xstate().
16 bytes boundary is OK for rt_sigframe.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>