bitops.h: sextract64() return type should be int64_t, not uint64_t

bitops.h: sextract64() return type should be int64_t, not uint64_t

The documentation for sextract64() claims that the return type is
an int64_t, but the code itself disagrees. Fix the return type to
conform to the documentation and to bring it into line with
sextract32(), which returns int32_t.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 1423231328-15662-1-git-send-email-peter.maydell@linaro.org