gdbserver: introduce support to show the AVX registers.

gdbserver: introduce support to show the AVX registers.

This implies to change the interface between the
arch independent gdbserver files and the arch dependent files
as AVX implies a choice of xml files at run time.

In valgrind-low-amd64.c, the xml files and the nr of registers
are different depending on AVX support or not.

Other platforms still have a fully static nr of registers.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12581