gdb: fully separate coff and elf reading from dbx

With the previous commits, the only thing entangling elf and coff file
reading with dbx file reading is the functions
{elf|coff}stab_build_psymtabs, defined in dbxread.c. These functions
depend on dbx_symfile_read.

To solve this, I renamed read_stabs_symtab to read_stabs_symtab_1, and
created a function with the original name that does what
dbx_symfile_read used to do.

This way, dbx_symfile_read can just call read_stabs_symtab, and the elf
and coff psymtab builders can also call it directly, fully disentangling
the readers, which would allow us to selectively not compile dbxread in
the future.

Approved-By: Tom Tromey <tom@tromey.com>