asm-generic/sembuf: Update architecture related information in comment

The structure came originally from x86_32 but is used by most of the
architectures now. Update the comment which says it is for x86 only.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/64efe033394b6f0dfef043a63fd8897a81ba6d16.1589970173.git.viresh.kumar@linaro.org'