Add macro definitions that make it possible to do cpu/os/both

Add macro definitions that make it possible to do cpu/os/both
ifdeffery in a sane way where it's absolutely unavoidable.  When
building the core, eg on amd64-linux, the following preprocessor
symbols are defined:

   VGA_amd64
   VGO_linux
   VGP_amd64_linux

etc on other platforms.

Also, include/basic_types.h now defines VG_WORDSIZE and this is what
should be used for ifdefs that need to know the host word size.

ifdefs based on the C compilers built-ins such as __amd64__ etc
are deprecated and will be done away with.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@3590