Recent Fuchsia SDKs have begun removing both symbols for unsupported
POSIX APIs, and also the relevant definitions, and even headers.

This CL:
- Removes dependencies on <sys/resource.h>.
- Adds a working implementation of GetUserTime().
- Fixes GetCurrentThreadId() to use the native (32-bit) Fuchsia thread
  handle, rather than the (64-bit) pthread*, to avoid potential for id
  clashes when truncating the value into a 32-bit int.

Bug: chromium:707030
Change-Id: Ic5774e138f7657123dd65d0fb7ef5d87876766e8
Reviewed-on: https://chromium-review.googlesource.com/933247Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51579}