Add valgrind headers to U-Boot

Valgrind uses magic code sequences to define an ABI that the client may use
to request behavior from the host. In particular, this may be used to
inform valgrind about custom allocators, such as the one used in U-Boot.

This adds headers defining these sequences to U-Boot. It also adds a config
option to disable emission of these sequences entirely, in the (likely)
event that the user does not wish to use valgrind. Note that this option is
called NVALGRIND upstream, but was renamed (and inverted) to
CONFIG_VALGRIND. Aside from this and the conversion of a few instances of
VALGRIND_DO_CLIENT_REQUEST_EXPR to STMT, these headers are unmodified.

These headers were copied from valgrind 3.16.1-4 as distributed in Arch
Linux. They are licensed with the bzip2 1.16 license. This appears to be a
BSD license with some clauses from Zlib.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>