Add support for a CPU only buffer for PBOs that serve as
the destination for all host operations like MapBuffer*.
This removes the latency caused by waiting for the in-flight
GPU commands to be complete before handing over the buffer
to the app.

This change removes a ~6ms wait/sleep on the first call to
MapBuffer* in each frame of Manhattan

Bug: angleproject:4339
Tests: angle_end2end_tests --gtest_filter=BufferDataTest*Vulkan
Change-Id: I52016b160af8a670cc30f01c05e48f699521310f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2116874
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>