Get start and end address of main in dwz.exp

Get start and end address of main in dwz.exp

On arm-none-eabi target thumb mode, I see the following fail,

p the_int^M
$2 = 99^M
(gdb) FAIL: gdb.dwarf2/dwz.exp: p the_int

and on powerpc64 target, we even can't get function main from object
file,

disassemble main^M
No function contains specified address.^M
(gdb) FAIL: gdb.dwarf2/dwz.exp: disassemble main

This patch is to use MACRO_AT_func attribute to get the main's start
address and end address correctly, and also remove some code dwz.exp
getting main's length.  This patch fixes fails on both thumb mode and
powerpc64 target.

PASS: gdb.dwarf2/dwz.exp: p other_int
PASS: gdb.dwarf2/dwz.exp: p the_int

gdb/testsuite:

2014-11-14  Yao Qi  <yao@codesourcery.com>

	* gdb.dwarf2/dwz.exp: Remove the code to compile main.c to
	object and get function length.
	(Dwarf::assemble): Replace name, low_pc and high_pc attributes
	with MACRO_AT_func.
	(top-level): Replace gdb_compile and clean_restart with
	prepare_for_testing.
	* gdb.dwarf2/main.c (main): Add label main_label.