[ldso][ldmsg] Remove DebugPrint from ldsvc; always use zx_log_write for now

We were already using zx_log_write for __sanitizer_log_write and it's
the only thing at hand that does good PID/thread/timestamp tagging.
The symbolizer needs the contextual elements emitted by dynlink.c and
the sanitizer logging to go the same route so they get consistent PID
tagging. So just use zx_log_write for all of it.

This should be replaced ASAP with a non-zx_log_write method for most
processes. Then _dl_log_write will again use zx_log_write only if the
process received PA_FDIO_LOGGER, as before.

ZX-2072 #done

Change-Id: I56e0290d92cf2a7efcc58a065dd61dc001b178a5

[ldmsg] Convert the loader service to FIDL wire format

Previously, the loader service used a hand-rolled protocol. Now the
protocol is specified in //zircon/system/public/zircon/loader.fidl.

Because this protocol is used by such low-level components (e.g.,
userboot and libc), we do not use generated code to implement the
protocol. Instead, we use a stand-alone library, ldmsg, which manually
encodes and decodes messages according to the wire format specified in
loader.fidl.

This CL also moves fidl/types.h into zircon/fidl.h because these type
definitions are now fundamental to the operation of the system. Putting
their definition in this location makes them easier to reference from
userboot and libc.

Change-Id: I9a7619b09b0ff543b8883ac53852ade9d5c033b6

Revert "[ldmsg] Convert the loader service to FIDL wire format"

This reverts commit 3ff2a3f316904d6acf709169fb12ee059231d5d9.

Additional review comments need to be addressed.

Original change's description:
> [ldmsg] Convert the loader service to FIDL wire format
>
> Previously, the loader service used a hand-rolled protocol. Now the
> protocol is specified in //zircon/system/public/zircon/loader.fidl.
>
> Because this protocol is used by such low-level components (e.g.,
> userboot and libc), we do not use generated code to implement the
> protocol. Instead, we use a stand-alone library, ldmsg, which manually
> encodes and decodes messages according to the wire format specified in
> loader.fidl.
>
> This CL also moves fidl/types.h into zircon/fidl.h because these type
> definitions are now fundamental to the operation of the system. Putting
> their definition in this location makes them easier to reference from
> userboot and libc.
>
> Change-Id: I806455813d3a397a2eff41badadf61c28812e328

TBR=kulakowski@google.com,swetland@google.com,abarth@google.com

Change-Id: I7ac90b53cdc152dc8cd892eb394ec7f641161ee9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true

[ldmsg] Convert the loader service to FIDL wire format

Previously, the loader service used a hand-rolled protocol. Now the
protocol is specified in //zircon/system/public/zircon/loader.fidl.

Because this protocol is used by such low-level components (e.g.,
userboot and libc), we do not use generated code to implement the
protocol. Instead, we use a stand-alone library, ldmsg, which manually
encodes and decodes messages according to the wire format specified in
loader.fidl.

This CL also moves fidl/types.h into zircon/fidl.h because these type
definitions are now fundamental to the operation of the system. Putting
their definition in this location makes them easier to reference from
userboot and libc.

Change-Id: I806455813d3a397a2eff41badadf61c28812e328