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

[Erik Joelsson]

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/

/Erik