target-arm: fix neon shift helper functions

target-arm: fix neon shift helper functions

Current code is broken at least on recent compilers, comparison
between signed and unsigned types yield incorrect code and render
the neon shift helper functions defunct. This is the third revision
of this patch, casting all comparisons with the sizeof operator to
signed ssize_t type to force comparisons to be between signed integral
types.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
Acked-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>