XS RFR: 8002040 Allow Full Debug Symbols when cross-compiling

[David Holmes]

webrev:

http://cr.openjdk.java.net/~dholmes/8002040/webrev/

This is a simplified variant of the change just made to Hotspot. Instead 
of disabling FDS when cross-compiling we change the default location of 
objcopy, and just as with native compilation either the default objcopy 
is found or we try to fallback to ALT_OBJCOPY if set. In other words the 
only difference between the two cases is the chosen default. (Arguably 
both cases could just use COMPILER_PATH/objcopy but that might change 
existing behaviour in some cases - which we do not want to do).

I will push this via the build repo. There are no corresponding changes 
needed to the new build as it handles FDS differently.

This only impacts Linux.

Thanks,
David