Move everything related to reading .gdb_index from read.c to
read-gdb-index.c. The only entry point exposed by read-gdb-index.{c,h}
is dwarf2_read_gdb_index.
Change-Id: I1e32c8f0720086538de8d2f612f27545377099bc
Approved-By: Tom Tromey <tom@tromey.com>