Remove duplicated code on checking address 0x0 is accessiable

Remove duplicated code on checking address 0x0 is accessiable

I find some gdb.python tests fail on arm-none-eabi target, because the
tests assume that memory on address 0x is inaccessible.  Some tests
(in gdb.base) are aware of this, so do a "x 0" check first.  However,
the code is copy-n-paste.

This patch is to move the "x 0" check to a procedure in lib/gdb.exp,
and get needed tests call it.  The original code matches pattern
"0x0:\[ \t\]*Error accessing memory address 0x0\r\n$gdb_prompt $", but
I remove it from the new proc is_address_zero_readable, because GDB
doesn't emit such message any more.

gdb/testsuite:

2014-08-09  Yao Qi  <yao@codesourcery.com>

	* gdb.base/display.exp: Invoke is_address_zero_readable.
	* gdb.guile/scm-value.exp (test_value_in_inferior): Likewise.
	* gdb.python/py-value.exp (test_value_in_inferior): Likewise.
	* gdb.base/hbreak-unmapped.exp: Return if
	is_address_zero_readable returns true.
	* gdb.base/signest.exp: Likewise.
	* gdb.base/signull.exp: Likewise.
	* gdb.base/sigbpt.exp: Likewise.
	* gdb.guile/scm-disasm.exp: Do the test if
	is_address_zero_readable returns false.
	* gdb.guile/scm-pretty-print.exp (run_lang_tests): Likewise.
	* gdb.python/py-arch.exp: Likewise.
	* gdb.python/py-prettyprint.exp (run_lang_tests): Likewise.
	* lib/gdb.exp (is_address_zero_readable): New proc.