gdb: remove COMPUNIT_FILETABS macro

gdb: remove COMPUNIT_FILETABS macro

I think that most remaining uses of COMPUNIT_FILETABS intend to get the
primary filetab of the compunit_symtab specifically (and not to iterate
over all filetabs, for example, those cases would use compunit_filetabs,
which has been converted to compunit_symtab::filetabs), so replace mosts
uses with compunit_symtab::primary_filetab.

In jit.c, function finalize_symtab, we can save the symtab object
returned by allocate_symtab and use it, it makes things simpler.

Change-Id: I4e51d6d4b40759de8768b61292e5e13c8eae2e38