Dwarf assembler: handle one instruction function

Dwarf assembler: handle one instruction function

On aarch64, we got the following fail:

(gdb) disassemble func
Dump of assembler code for function func:
   0x0000000000400730 <+0>:     ret
End of assembler dump.^M
(gdb) x/2i func+0^M
   0x400730 <func>:     ret^M
   0x400734 <main>:     stp     x29, x30, [sp,#-16]!^M
(gdb) FAIL: gdb.dwarf2/dw2-ifort-parameter.exp: x/2i func+0

the pattern in proc function_range expects to match <func+0>, however,
GDB doesn't display the offset when it is zero.  This patch is to
adjust the pattern when $func_length is zero.

gdb/testsuite:

2015-02-26  Yao Qi  <yao.qi@linaro.org>

	* lib/dwarf.exp (function_range): Adjust pattern when $func_length
	is zero.