[build] Make a number of static libraries build with -fvisibility=hidden

ZX-1895 #comment
Test: CQ

Change-Id: Id3252fcb1de5a6f6e1897da86fba92e2be9d0319

[debugger] Don't export task-utils as public.

This was recently added in
https://fuchsia-review.googlesource.com/c/zircon/+/125480
but this ended up not being very helpful for my use-case. Since it's no longer
required to be public, this marks it private until we need it again.

Change-Id: Iddb739c0af26d8eadd8c4bcbbe5565709745be43

[debugger] Export ulib/task-utils as public.

This will be used to fetch the process and thread trees as part of the
debugger.

This code is exported as a static library because it's fairly little code and
likely to be not commonly used enough to justify shipping as a separate .so.

Update readme to say the files are generated automatically (I tried to do it
manually for a few minutes since it looked like these were in the source tree).

Change-Id: Id8b13a21876bbf6384918be40ab404d0928be24b

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[ulib][task-utils] Add a C++ task-walker interface and tests

Change-Id: I5ef848e521560a8744226e7239b79fffc076b46a

[ulib][task-utils] Add get_task_by_koid()

Change-Id: I32866eb376539d2f58912d271966c91bcd28f1b0

[ulib] Move psutil's processes.c into ulib/task-utils

- Clean up the API a little
- Call the callback on the root job, and find its koid
- Don't walk processes/threads if no callback is provided

Change-Id: Ia77cc84b5aee4ef331ec28054d36ae0aad67e5a0