[Bug libstdc++/93421] futex.cc use of futex syscall is not time64-compatible

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93421

--- Comment #2 from Rich Felker <bugdal at aerifal dot cx> ---
Rather than #if defined(SYS_futex_time64), I think it should be made:

#if defined(SYS_futex_time64) && SYS_futex_time64 != SYS_futex

This is in consideration of support for riscv32 and future archs without legacy
syscalls. It's my intent in musl to accept the riscv32 port with SYS_futex
defined to be equal to SYS_futex_time64; otherwise all software making use of
SYS_futex gratuitously breaks.