Compute the function length instead of hard coding it

In Dwarf::assemble in dwz.exp, 10 is hard-coded in it,

 	    subprogram {
 		{name main}
 		{low_pc main addr}
		{high_pc "main + 10" addr}
 	    }

however, the length of main function varies on architectures.  The
hard-coded 10 here causes dwz.exp fails on some targets, such as
nios2.

This patch is to add some code to compute the length of function main,
which is similar to what we are doing in entry-values.exp.

gdb/testsuite:

2014-04-26  Yao Qi  <yao@codesourcery.com>

	* gdb.dwarf2/dwz.exp: Compile main.c to object.  Restart GDB
	and compute the length of function main.  Save it in
	$main_length.
	(Dwarf::assemble): Use $main_length instead of hard-coded 10.
	(top-level): Use gdb_compile to compile objects into
	executable and restart GDB.  Remove invocation to
	prepare_for_testing.