_WIN32 is defined for 64-bit builds as well, so test for _WIN64 first.

This requires pointing it at the valgrind headers we carry in-tree.

I was still having issues with the build system somehow getting confused
as to which set of valgrind headers to use when compiling rt.

This commit moves all the valgrind headers into their own directory
under rt and makes the usage more consistent. The compiler is now passed
the -DNVALGRIND flag when valgrind is not installed, as opposed to
passing -DHAVE_VALGRIND.

We also pass -I src/rt to the compiler when building rt so you can more
easily import what you want. I also cleaned up some erroneous #includes
along the way.

It should be safe to always just import the local valgrind headers and use
them without question. NVALGRIND turns the operations to no-ops when it
is active, and the build and tests run cleanly with or without.

This applies c11675 by Julian Seward from valgrind trunk
(svn://svn.valgrind.org/valgrind/trunk) to rustrt's included
copies of memcheck.h and valgrind.h, effectively backporting
the fix from the unreleased 3.6.2. The commit simply applies
the gcc 'unused' attribute to the relevant declarations.

This change allows compilation of the runtime code under
gcc 4.6 with -Werror, as the makefile currently requests.