[fbl] Remove zx dependency

This dependency isn't needed anymore.

Test: Compiles.
Change-Id: I286b630d55ab0be71baadda0cb56ccc7fc3cf59b

[fbl] Remove libzx dependency from libfbl

Code that depends on the Zircon syscalls should be in fzl instead.
See <https://fuchsia.googlesource.com/zircon/+/master/docs/cxx.md#fzl>

Test: fzl-test
Change-Id: Ic65425fc51136edf4d401097f6b483d56923104e

[ulib][fbl][zx] Build fbl and zx with -fvisibility=hidden

This is the first incremental change of a migration that will
culminate in adding -fvisibility=hidden to GLOBAL_COMPILEFLAGS
and removing all the MODULE_COMPILEFLAGS additions.

As these two libraries are static-only, there are no __EXPORT
markers to add.

ZX-1895 #comment Converted fbl and zx

Change-Id: I7c1c37b6a21c6e80af0eeafea4953201ad0d3d48

[fbl][vmo] Moving VmoMapper out of fbl

The VmoMapper and the VmarManager classes are more
of vmo utilities than fbl libraries. In preparation
for adding more such utilities, I am moving these libraries
to another location.

There are other CLs to update usage of this library in
the other repos.

Change-Id: I610bfd2838cf7843120d4aa885d24ea3b1b97b73

[tests] Add probe_for_read|write test helpers.

These allow testing whether a memory address is readable or writable.

ZX-1574

Change-Id: I630a0f6fb9177f14ab13918da7e5ec7ed2fc584f

[fbl] Add a "VmoMapper" utility class.

Add a small class which handles the process of creating a VMO of a
given size and mapping it into the local address space, as well as
automatically unmapping at end-of-life.

Change-Id: I10fba0ae90201a414a0a49eab4c77150081bec2c

[build] start exporting package descriptions for source packages

Change-Id: Idd4e916dcfee4d9f40cbb82827042b746c8ce987

[fbl][string_buffer] Add AppendPrintf functions.

Change-Id: I470299999d68cb8880b0b2831c916b6f3fa6e68d

[zircon][mxtl->fbl] Rename mxtl to fbl

Change-Id: Ie21b6498e1bfb0a7fa0315e40b9e5c3ee78646be