Create dwarf2/leb.[ch]

Create dwarf2/leb.[ch]

This moves some scalar-unpacking code into a couple of new files,
dwarf2/leb.h and dwarf2/leb.c.

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

	* dwarf2read.h (read_unsigned_leb128): Don't declare.
	* dwarf2read.c (read_1_byte, read_1_signed_byte, read_2_bytes)
	(read_2_signed_bytes, read_3_bytes, read_4_bytes)
	(read_4_signed_bytes, read_8_bytes): Move to dwarf2/leb.h.
	(read_unsigned_leb128, read_signed_leb128): Move to dwarf2/leb.c.
	* dwarf2/leb.h: New file, from dwarf2read.c.
	* dwarf2/leb.c: New file, from dwarf2read.c.
	* dwarf2-frame.c (read_1_byte, read_4_bytes, read_8_bytes):
	Remove.
	* Makefile.in (CONFIG_SRC_SUBDIR): Add dwarf2.
	(COMMON_SFILES): Add dwarf2/leb.c.

Change-Id: Idd19647686c8f959d226a95fdfca4db47c6e96d0