gdb: handle invalid DWARF when compilation unit is missing

gdb: handle invalid DWARF  when compilation unit is missing

Replace an abort call in process_psymtab_comp_unit with a real error,
and add a test to cover this case.  The case is question is when badly
formed DWARF is missing a DW_TAG_compile_unit, DW_TAG_partial_unit, or
DW_TAG_type_unit as its top level tag.

I then tested with --target_board=readnow and added additional code to
also validate the top-level tag in this case.

I added an assert that would trigger for the readnow case before I
added the fix.  I suspect there's lots of places where badly formed
DWARF could result in the builder being nullptr when it shouldn't be,
but I only added this one assert, as this is the one that would have
helped me in this case.

gdb/ChangeLog:

	* dwarf2/read.c (process_psymtab_comp_unit): Replace abort with an
	error.
	(process_full_comp_unit): Validate the top-level tag before
	processing the first DIE.
	(read_func_scope): Ensure we have a valid builder.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-missing-cu-tag.c: New file.
	* gdb.dwarf2/dw2-missing-cu-tag.exp: New file.