SPARC: optimized interrupt stack frame size

With this change we allocate stack space only for the registers we
actually store in the thread interrupt stack frame.

Furthermore, no function is called on with the interrupt context save
frame %sp so no full frame is needed here. ABI functions are called
later in the interrupt trap handler, but that is after the dedicated
interrupt stack has been installed.

This saves 96 bytes of stack space for each interrupted context.

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>