RFR (S): 8074726: Update source and target version used when compiling hotspot class files

[Mikael Vidstedt]

The Java class files under hotspot/ are currently compiled using 
"-source 6 -target 6" which generates unnecessarily old class files and 
also prints out warning messages doing so. This change does two things:

* bumps the default version used to "-source 8 -target 8"
* uses the BOOT_JDK_SOURCETARGET from the top level build system if it 
has been set (typically to "-source 8 -target 8")

Tested through JPRT, the serviceability/sa regression tests as well as 
some Oracle internal tests.

http://cr.openjdk.java.net/~mikael/webrevs/8074726/webrev.00/webrev/

The change in make/defs.gmk is needed to pass the relevant option 
through to the jvmti docs compilation.

Cheers,
Mikael