[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