[gdb/symtab] Enable ada .gdb_index

[gdb/symtab] Enable ada .gdb_index

Currently the .gdb_index is not enabled for ada executables (PR24713).

Fix this by adding the required support in write_psymbols, similar to how that
is done for .debug_names in debug_names::insert.

Tested on x86_64-linux, with native and target board cc-with-gdb-index.

gdb/ChangeLog:

2020-06-10  Tom de Vries  <tdevries@suse.de>

	PR ada/24713
	* dwarf2/index-write.c (struct mapped_symtab): Add m_string_obstack.
	(write_psymbols): Enable .gdb_index for ada.
	* dwarf2/read.c: Remove comment stating .gdb_index is unsupported for
	ada.

gdb/testsuite/ChangeLog:

2020-06-10  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/ptype_union.exp: Remove PR24713 workaround.