[libcxx] Use Fuchsia-native monotonic clock for std::chrono::steady_clock

Use the zx_clock_get_monotonic system call directly rather than
going through the POSIX clock_gettime function.  The libc function
is a trivial wrapper around the system call, and is not a standard C
function.  Avoiding it reduces the Fuchsia libc ABI surface that
libc++ depends on.

Reviewed By: phosek, ldionne, #libc

Differential Revision: https://reviews.llvm.org/D116606