Harden gdb.base/bp-permanent.exp

This testcase does not work as expected in QEMU (aarch64 QEMU in my case). It
fails when trying to manually write the breakpoint instruction to a certain
PC address.

(gdb) p /x addr_bp[0] = buffer[0]^M
Cannot access memory at address 0x400834^M
(gdb) PASS: gdb.base/bp-permanent.exp: always_inserted=off, sw_watchpoint=0: setup: p /x addr_bp[0] = buffer[0]
p /x addr_bp[1] = buffer[1]^M
Cannot access memory at address 0x400835^M
(gdb) PASS: gdb.base/bp-permanent.exp: always_inserted=off, sw_watchpoint=0: setup: p /x addr_bp[1] = buffer[1]
p /x addr_bp[2] = buffer[2]^M
Cannot access memory at address 0x400836^M
(gdb) PASS: gdb.base/bp-permanent.exp: always_inserted=off, sw_watchpoint=0: setup: p /x addr_bp[2] = buffer[2]
p /x addr_bp[3] = buffer[3]^M
Cannot access memory at address 0x400837^M
(gdb) PASS: gdb.base/bp-permanent.exp: always_inserted=off, sw_watchpoint=0: setup: p /x addr_bp[3] = buffer[3]

The following patch prevents a number of failures by detecting this and bailing out in case the target has such a restriction. Writing to .text from within the program isn't any better. It just leads to a SIGSEGV.

Before the patch:

                === gdb Summary ===

After the patch:

                === gdb Summary ===

gdb/testsuite/ChangeLog:
2015-04-13  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.base/bp-permanent.exp (test): Handle the case of being unable
	to write to the .text section.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2015-04-13  Luis Machado  <lgustavo@codesourcery.com>
+
+	* gdb.base/bp-permanent.exp (test): Handle the case of being unable
+	to write to the .text section.
+
 2015-04-13  Luis Machado  <lgustavo@codesourcery.com>
 
 	* gdb.base/coredump-filter.exp: Restrict test to Linux systems only.

gdb/testsuite/gdb.base/bp-permanent.exp

@@ -104,7 +104,18 @@ proc test {always_inserted sw_watchpoint} {
 	# to memory manually.
 	set count [expr $address_after_bp - $address_bp]
 	for {set i 0} {$i < $count} {incr i} {
-	    gdb_test "p /x addr_bp\[$i\] = buffer\[$i\]" " = .*"
+	    gdb_test_multiple "p /x addr_bp\[$i\] = buffer\[$i\]" $test {
+		-re "Cannot access memory at address $hex.*$gdb_prompt $" {
+		    # Some targets (QEMU for one) will disallow writes to the
+		    # .text section under certain circumstances.  It is no use
+		    # continuing with the test at this point.  Just return.
+		    unsupported "Cannot modify memory"
+		    return
+		}
+		-re " = .*$gdb_prompt $" {
+		    pass $test
+		}
+	    }
 	}
     }