[XRay][compiler-rt][Darwin] Minimal XRay build support in Darwin

This change is the first in a series of changes to get the XRay runtime
building on macOS. This first allows us to build the minimal parts of
XRay to get us started on supporting macOS development. These include:

  - CMake changes to allow targeting x86_64 initially.
  - Allowing for building the initialisation routines without
    `.preinit_array` support.
  - Use __sanitizer::SleepForMillis() to work around the lack of
    clock_nanosleep on macOS.
  - Deprecate the xray_fdr_log_grace_period_us flag, and introduce
    the xray_fdr_log_grace_period_ms flag instead, to use
    milliseconds across platforms.

Reviewers: kubamracek

Subscribers: llvm-commits, krytarowski, nglevin, mgorny

Differential Review: https://reviews.llvm.org/D39114

llvm-svn: 319165

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