[REDO] Fix sun.awt.nativedebug on X11 platforms

AWT supports some kind of native logging which can be enabled with "-Dsun.awt.nativedebug=true -Dawtdebug.ctrace=true".

Unfortunately this doesn't work on X platforms any more because both, libawt and libawt_xawt end up with a copy of debug_trace.o. Among others, debug_trace.o contains the static variable GlobalTracingEnabled which denotes the tracing state. This obviously can't work if the final executable contains several instances of this variable.

The fix is trivial. Remove "common/awt/debug" from the set of sources for libawt_xawt. libawt_xawt is linked against libawt (which contains debug_trace.o) anyway, so this is no problem.

Also JNIEXPORT the following methods from libawt.so otherwise they won't be visible in libawt_xawt.so on some platforms which make symbols local by default (e.g. Solaris):

DAssert_Impl
DTrace_PrintFunction
DTrace_VPrint
DTrace_VPrintln