[zircon][syscalls] Make zx_time_t and zx_duration_t signed types

This change is part of a hard transition and breaks the layers above.

zx_time_t and zx_duration_t are now int64_t. ZX_TIME_INFINITE is now
INT64_MAX.

A negative time value passed to a syscall is treated as any other time
value that's "in the past".

Test: CQ; new unit tests; built and ran topaz system on Eve

ZX-2100 #comment

Change-Id: I2a020cb2cf3dd7212608e24c1ee90cec58dd51ef

[kernel][crash] switch to symbolizer format

Format at https://fuchsia.googlesource.com/zircon/+/master/scripts/symbolize#314

DX-230

TESTED=`k crash` on device (arm64 and x64) then `fx syslog | fx symbolize` upon reboot

Change-Id: I1a11401348e774b01304c4996d518d247b954237

[kernel][crash] only dump iframe if non-NULL

ZX-2476 #done

TESTED=assert(false) at the top of system/core/devmgr/devmgr.c's main() does not lead to a panic anymore on QEMU

Change-Id: I654c09c0b456d824b7e1b8c8e696dc9b877bfc27

[kernel][crashlog] move away from printf callback

instead of piggybacking on the printf messages, we build a crashlog
struct with the relevant fields that we then serialize to string

DX-230 # step 1

TESTED=manual run of `k crash` on device (arm64 and x64) and manual inspection of logs upon reboot

Change-Id: I6019c85cbd0d71084d543667850dea30ac06ea93