linux-user: avoid gcc array overrun warning for sparc

Suppress a gcc array bounds overrun warning when filling in the SPARC
signal frame by adjusting our definition of the structure so that the
fp and callers_pc membes are part of the ins[] array rather than
separate fields; since qemu has no need to access the fields individually
there is no need to follow the kernel's structure field naming exactly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
