linux-user: Correct definition of stack_t

linux-user: Correct definition of stack_t

Some platforms used the wrong definition of stack_t where the flags and
size fields were swapped or where the flags field had type ulong instead
of int.

Due to the presence of padding space in the structure and the prevalence
of little-endian machines this problem went unnoticed for a long time.

The type definitions have been cross-checked with the ones defined in
the Linux kernel v5.9, plus some older versions for a few architecture
that have been removed and Xilinx's kernel fork for NiosII [1].

The bsd-user headers remain unchanged as I don't know if they are wrong
or not.

[1] https://github.com/Xilinx/linux-xlnx/blob/master/arch/nios2/include/uapi/asm/signal.h

Signed-off-by: Giuseppe Musacchio <thatlemon@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <e9d47692-ee92-009f-6007-0abc3f502b97@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>