[zircon][kernel] Use proper time types and overflow-safe arithmetic

This change is preparation for converting zx_time_t and zx_duration_t
from unsigned to signed types (ZX-2100).

1. Fix several places where the wrong time type was used (e.g.
zx_time_t instead of zx_duration_t). Because these are simple typedefs
it's easy to use the wrong one. While it generally doesn't cause a
problem, using the wrong type can be misleading and lead to bugs like
passing a relative time when an absolute time is expected.

2. Update kernel/ and some other parts of zircon to use the new
overflow-safe arithmetic functions from zircon/time.h.

Performance: Ran "/pkgfs/packages/zircon_benchmarks/0/bin/app -p" on
NUC before and after this change. Differences were withing the noise,
some slightly faster, some slightly slower.

Test: CQ

ZX-2100 #comment cleanup before conversion

Change-Id: I75d074e5850ade5385decd9bbf330497bf7610e9

[sync] Apply C API rubric to libsync

Test: No behavior change
Change-Id: I30a413f7fb5d3f4e741e5204532b82e73595c086

[dev][intel-hda][dsp] use FW_READY ipc notification

Wait for both the FW_READY IPC notification and fw_status
when waiting for the DSP to load firmware.

Change-Id: I35edb74f31ab20e3756bce529f4b7f1b52dd782b

[dev][intel-hda][dsp] introduce ipc workqueue

Change-Id: Ia1aeefd92fae121341a0974b005b0167d91da154

[dev][intel-audio-dsp] initialize basic DSP pipeline

Initialize a single pipeline for speaker playback.

Change-Id: I7eea457e4a53a804942924b90c1bbf35b833a5f1