Delay allocation of dbx_symfile_info

I noticed that elfread.c always allocates a dbx_symfile_info, even
though this is only ever needed in the unusual case of reading stabs
in ELF.

This patch moves the allocation into dbxread.c, and applies the same
treatment to similar code in coffread.c.

Regression tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-06-10  Tom Tromey  <tromey@adacore.com>

	* elfread.c (elf_read_minimal_symbols): Don't set the dbx objfile
	data.
	(elf_new_init): Don't call stabsread_new_init.
	* dbxread.c (coffstab_build_psymtabs): Set dbx objfile data.
	(elfstab_build_psymtabs): Likewise.  Call stabsread_new_init.
	* coffread.c (coff_symfile_init): Don't set the dbx objfile data.