RFR: JDK-8224677: Dtrace .d files clash with make dependency .d files

[Tim Bell]

Erik:

> Since JDK-8217728, where I introduced some optimizations for handling of
> the generated make dependency files for native compilation, building
> hotspot incrementally on Solaris has been broken. The reason for the
> breakage is that there is a clash between the *.d (header) files created
> by the dtrace build and the *.d (make) files created by the compiler.
> The optimization was to cat all the *.d make file together into a single
> file, but if dtrace is enabled, this cat will inadvertently include the
> dtrace files as well, which causes parse errors in make when make tries
> to include the resulting file.
>
> This fix changes the file extension used by dtrace files in the output
> dir to *.dt to avoid the clash.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8224677
>
> Webrev: http://cr.openjdk.java.net/~erikj/8224677/webrev.01/

So the .dt files are temporary artifacts in the build anyway, used while 
creating dtrace.o.  Looks good.

/Tim