[sanitizer][fuchsia] Avoid deprecated syscall.

The zx_clock_get syscall on Fuchsia is deprecated - ref
https://fuchsia.dev/fuchsia-src/reference/syscalls/clock_get
This changes to the recommended replacement; calling zx_clock_read on
the userspace UTC clock.

Reviewed By: mcgrathr, phosek

Differential Revision: https://reviews.llvm.org/D90169