Debugger: Add infrastructure for loading Symbian .sym files.

Add local symbol files for libraries to gdb along with
section addresses using the command
add-symbol-file <file> <address[es]>.

Whenever TRK reports a module load, check for a matching local
.sym file in a folder pointed to by the
QTC_SYMBIAN_SYMBOLFILE_CACHE environment variable
or the folder of the application executable symbol file (which
should be udeb/urel of EPOC).

Rubber-stamped-by: hjk