Shrink size of dwarf2_per_cu_data

Shrink size of dwarf2_per_cu_data

I noticed some holes in struct dwarf2_per_cu_data.  This patch
rearranges the type slightly, and shrinks the size of some fields.
This reduces it from 136 bytes to 112 bytes (on x86-64).

I also reduced the size of the DWARF "version" fields in a couple of
spots.  It seemed needless to use a short to hold a value that ranges
from 2 to 5, and this also helped the goal of shrinking
dwarf2_per_cu_data.

2021-04-21  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (struct dwarf2_per_cu_data) <dwarf_version>: Now
	unsigned char.
	(struct dwarf2_per_cu_data): Rearrange.
	* dwarf2/comp-unit.h (struct comp_unit_head) <version>: Now
	unsigned char.
	(struct comp_unit_head): Rearrange.
	* dwarf2/comp-unit.c (read_comp_unit_head): Update.