Fix implptr-optimized-out.exp fail

Fix implptr-optimized-out.exp fail

Hi,
I see the fail in gdb.dwarf2/implptr-optimized-out.exp in thumb mode

(gdb) p p->f^M
No symbol "p" in current context.^M
(gdb) FAIL: gdb.dwarf2/implptr-optimized-out.exp: p p->f

and the crash on powerpc64

(gdb) continue^M
Continuing.^M
^M
Program received signal SIGSEGV, Segmentation fault.^M
0x7d82100810000828 in ?? ()

The cause of both is that we incorrectly set attribute low_pc, since
main isn't resolved to function start address on these targets.

In this patch, we replace attributes name, low_pc and high_pc with
MACRO_AT_func.  The fail on thumb mode is fixed, and crash on
powerpc64 is fixed too.

gdb/testsuite:

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

	* gdb.dwarf2/implptr-optimized-out.exp (Dwarf::assemble):
	Replace name, low_pc and high_pc with MACRO_AT_func.