Prepare for AVX support : restructure gdbsrv/target/valgrind-low/arch low

Prepare for AVX support : restructure gdbsrv/target/valgrind-low/arch low

AVX support implies to have target xml files which are selected
according to the machine hwcaps.
This change improves the structure of the gdbserver software layering
to prepare for this.

Basically, the protocol files (e.g. server.c) are now calling directly
the valgrind target operations which are now defined in target.h/target.c
(before, there was a level of indirection inheritated from the GDB
structure which was useless for valgrind gdbserver).
+ clarified some comments

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12579