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.

gdb/testsuite/gdb.python/py-arch.exp

@@ -57,6 +57,8 @@ gdb_test "python print (\"addr\" in insn)" "True" "test key addr"
 gdb_test "python print (\"asm\" in insn)" "True" "test key asm"
 gdb_test "python print (\"length\" in insn)" "True" "test key length"
 
-# Negative test
-gdb_test "python arch.disassemble(0, 0)" ".*gdb\.MemoryError.*" \
-  "test bad memory access"
+if { ![is_address_zero_readable] } {
+    # Negative test
+    gdb_test "python arch.disassemble(0, 0)" ".*gdb\.MemoryError.*" \
+	"test bad memory access"
+}

gdb/testsuite/gdb.python/py-prettyprint.exp

@@ -83,7 +83,10 @@ proc run_lang_tests {exefile lang} {
 	gdb_py_test_silent_cmd "set print elements 200" "" 1
     }
 
-    gdb_test "print ns2" ".error reading variable: Cannot access memory at address 0x0."
+    if { ![is_address_zero_readable] } {
+	gdb_test "print ns2" \
+	    ".error reading variable: Cannot access memory at address 0x0."
+    }
 
     gdb_test "print x" " = \"this is x\""
     gdb_test "print cstring" " = \"const string\""

gdb/testsuite/gdb.python/py-value.exp

@@ -234,14 +234,7 @@ proc test_value_in_inferior {} {
   # Test displaying a variable that is temporarily at a bad address.
   # But if we can examine what's at memory address 0, then we'll also be
   # able to display it without error.  Don't run the test in that case.
-  set can_read_0 0
-  gdb_test_multiple "x 0" "memory at address 0" {
-      -re "0x0:\[ \t\]*Cannot access memory at address 0x0\r\n$gdb_prompt $" { }
-      -re "0x0:\[ \t\]*Error accessing memory address 0x0\r\n$gdb_prompt $" { }
-      -re "\r\n$gdb_prompt $" {
-	  set can_read_0 1
-      }
-  }
+  set can_read_0 [is_address_zero_readable]
 
   # Test memory error.
   set test "parse_and_eval with memory error"