libxshmfence: Build fixes for riscv32

NR_futex is not defined by newer architectures e.g. riscv32 as
they only have 64bit variant of time_t. Glibc defines SYS_futex
interface based on __NR_futex, since this is used in applications,
such applications start to fail to build for these newer architectures.

Define a fallback to alias __NR_futex to __NR_futex_time64 to make
SYS_futex keep working.

Reference: https://git.openembedded.org/openembedded-core/commit/?id=7a218adf9990f5e18d0b6a33eb34091969f979c7



Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 81599bf3)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>