Remove a use of the comp_unit backlink

The DWARF frame comp_unit object still has a backlink to the objfile.
In order to be truly objfile-independent, this must be removed.

This patch removes one such use, by passing the gdbarch to
decode_frame_entry directly.

gdb/ChangeLog
2020-02-12  Tom Tromey  <tom@tromey.com>

	* dwarf2/frame.c (decode_frame_entry_1): Add gdbarch parameter.
	(decode_frame_entry): Likewise.
	(dwarf2_build_frame_info): Update.