gdb: remove SYMTAB_COMPUNIT macro, add getter/setter

gdb: remove SYMTAB_COMPUNIT macro, add getter/setter

Add a getter and a setter for a symtab's compunit_symtab.  Remove the
corresponding macro and adjust all callers.

For brevity, I chose the name "compunit" instead of "compunit_symtab"
the the field, getter and setter names.  Since we are already in symtab
context, the _symtab suffix seems redundant.

Change-Id: I4b9b731c96e3594f7733e75af1e3d01bc0e4fe92