Generate & concatenate dtrace hooks.

This removes the need to manually escape every newline when defining hook macros
for DTrace/Systemtap. This in turn makes them quite a bit easier to read.

Using this setup is quite simple: dump a header file in vm/dtrace/hooks (using
.h as the extension) and it will be concatenated into vm/dtrace/hooks.h. That
header is in turn included into vm/dtrace/dtrace.h.

This commit also moves the dummy METHOD_HOOK macro into probes_dummy.h. There's
no particular reason to keep it in dtrace.h.