Renamed and retyped the fields relating to valgrind's stack in os_state_t to

Renamed and retyped the fields relating to valgrind's stack in os_state_t to
make their role clearer and their behaviour more consistent with the fields
describing the client's stack.  Also made the code in x86-linux/syscalls.c
and amd64-linux/syscalls.c more word-size-independent, which is not strictly
necessary but makes the code similarities between the two files more
obvious.

One consequence of this is that Valgrind's stack on AMD64 is now 16384 * 8
bytes, rather than 16384 * 4 bytes.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@3520