[libc++] Avoid <climits> dependency in <thread>

The standard guarantees sleep durations of 2^63-1 nanoseconds to work.
Instead of depending on INT64_MAX or ULONGLONG_MAX to exist via the
header pollution, fold the constant directly. That has the additional
positive side effect that it avoids long double arithmetic bugs in GCC.

Differential Revision: https://reviews.llvm.org/D99516