Move read_offset_1 to leb.c

Move read_offset_1 to leb.c

This moves read_offset_1 to leb.c, as it is a low-level data-reading
function.  It is also renamed to remove the "_1", because gdb can use
overloading now, and this is clearer.

2020-02-08  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (read_offset_1): Move to leb.c.
	(read_abbrev_offset, read_offset, dwarf_decode_line_header)
	(dwarf_decode_macro_bytes): Update.
	* dwarf2/leb.c (read_offset): Rename; move from read.c.
	* dwarf2/leb.h (read_offset): Declare.

Change-Id: I048140598acfa76eade2cc529ab7933d4b9ca0b3