Wrap locally used classes in anonymous namespace

Wrap locally used classes in anonymous namespace

Both aarch64-tdep.c and arm-tdep.c defines a class instruction_reader, which
violates ODR, but linker doesn't an emit error.  I fix this issue by wrapping
them by anonymous namespace, but I think it is better to apply this for all
locally used classes.

If it is a good idea to put locally used class into anonymous namespace, we
should document this rule into GDB coding convention, or even GCC coding
convention.  Note that anonymous namespace has been used in GCC but GCC
coding convention doesn't mention the it.

gdb:

2017-03-22  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c: Wrap locally used classes in anonymous
	namespace.
	* arm-tdep.c: Likewise.
	* linespec.c: Likewise.
	* ui-out.c: Likewise.