For some targets __dso_handle will never be used, and its definition in
crtstuff.c can cause a domino effect resulting in the size of the final
executable increasing much more than just the size of this piece of data.

For msp430, CRT functions to initialize global data are only included if there
is global data to initialize and it is more than feasible to write functional
programs which do not use any global data. In these cases, the definition of
__dso_handle will cause code size to increase by around 100 bytes as library
code to initialize data is linked into the executable.

Removing __dso_handle can therefore save on code size.

This does require the target to NOT use __cxa_atexit, so either
TARGET_CXX_USE_ATEXIT_FOR_CXA_ATEXIT must return true or -fno-use-cxa-atexit
must be used as a target flag when building GCC.
This is because __cxa_atexit includes functionality to unload dynamic shared
objects and so cp/decl.c will create a reference to __dso_handle to facilitate
this in programs with static destructors.