[mxcpp] Add __cxa_thread_atexit

This necessary to support thread_local variables with destructors.

Change-Id: Ic3651fe631e16443b2e89ccec986770b5a4ba991

[build][ulib][musl] Fix build for C++ static destructors

Having libc export __dso_handle is multiple kinds of wrong.
The status quo ante caused linking a DSO that uses a C++
static destructor to fail with obscure errors.

Change-Id: I37c2a1913719246a741996c40c4c189efe3b3681

Revert "[build][ulib][musl] Fix build for C++ static destructors"

This reverts commit 37db1257d7407d1c5ed6c5dff7b94b931258d6b9.

Reason for revert:
The Fuchsia Clang toolchain fails to include an equivalent to
GCC's crtbegin.o, which is where __dso_handle ought to be defined.
We'll re-land this change once the toolchain is fixed.

Change-Id: I6f6319188dfc9ba46e666e653adc0b501c443f2e

[build][ulib][musl] Fix build for C++ static destructors

Having libc export __dso_handle is multiple kinds of wrong.
The status quo ante caused linking a DSO that uses a C++
static destructor to fail with obscure errors.

Change-Id: I801b948ba620d30f5eea908d0f43b51bf9d2e881

[system] align license and license banners with Fuchsia

This adjusts the Magenta userspace code to match the prevailing
license in Fuchsia (three-clause BSD), using the same banner
and license text.

Change-Id: I6b6d79435c5dbaa32027888874849e4f47aaa7be

[system][utest]/[mxsh] Make runtests output less by default

To output the full amount use "runtests -v".

This introduces a verbosity_level concept for system/ulib/unnittest
with unittest_set_verbosity_level(level) to control it.

Change-Id: I0f301adf4fdac9a3cf6c76e41d4b9587c9495d1f

[ulib][unittest] Move unittest into its own library

Change-Id: I1586278e3bbb6ceacd26a1342985922dc8861cf7

[build][userspace] Support .init_array for userspace with gcc

In particular, with this change, C++ constructors are running now, and
static C variables are initialized.

Change-Id: If0252773faae705af78902f782aaa200d0c5f85e