Don't print symbol on address 0x0

Don't print symbol on address 0x0

We see the following fails on arm-none-eabi target,

print (void*)v_signed_char^M
$190 = (void *) 0x0 <_ftext>^M
(gdb) FAIL: gdb.base/exprs.exp: print (void*)v_signed_char (print
(void*)v_signed_char)

GDB behaves correctly but the test assumes there is no symbol on
address 0x0.  This patch is set print symbol off, so that tests below
can match the address only.

gdb/testsuite:

2014-07-11  Yao Qi  <yao@codesourcery.com>

	* gdb.base/exprs.exp: "set print symbol off".