[MIPS] signal: clean up sigframe structure

This patch makes 'struct sigframe' declaration avalaible for all signals
code. It allows signal32 to not have its own declaration.

This patch also removes all ICACHE_REFILLS_WORKAROUND_WAR tests in
structure declaration and hopefully make them more readable.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>