x86: ia32_signal: rename struct sigframe and rt_sigframe

Impact: cleanup, prepare to include sigframe.h

Rename struct sigframe to struct sigframe_ia32 and struct rt_sigframe to
struct rt_sigframe_ia32.

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