watchpoint-reuse-slot.exp: skip setting HW breakpoints on some address

We see some fails in watchpoint-reuse-slot.exp on aarch64-linux, because
it sets some HW breakpoint on some address doesn't meet the alignment
requirements by kernel, kernel will reject the
ptrace (PTRACE_SETHBPREGS) call, and some fails are caused, for example:

(gdb) PASS: gdb.base/watchpoint-reuse-slot.exp: always-inserted off: watch x hbreak: : width 1, iter 0: base + 0: delete $bpnum
hbreak *(buf.byte + 0 + 1)^M
Hardware assisted breakpoint 80 at 0x410a61^M
(gdb) PASS: gdb.base/watchpoint-reuse-slot.exp: always-inserted off: watch x hbreak: : width 1, iter 0: base + 1: hbreak *(buf.byte + 0 + 1)
stepi^M
Warning:^M
Cannot insert hardware breakpoint 80.^M
Could not insert hardware breakpoints:^M
You may have requested too many hardware breakpoints/watchpoints.^M
^M
(gdb) FAIL: gdb.base/watchpoint-reuse-slot.exp: always-inserted off: watch x hbreak: : width 1, iter 0: base + 1: stepi advanced

hbreak *(buf.byte + 0 + 1)^M
Hardware assisted breakpoint 440 at 0x410a61^M
Warning:^M
Cannot insert hardware breakpoint 440.^M
Could not insert hardware breakpoints:^M
You may have requested too many hardware breakpoints/watchpoints.^M
^M
(gdb) FAIL: gdb.base/watchpoint-reuse-slot.exp: always-inserted on: watch x hbreak: : width 1, iter 0: base + 1: hbreak *(buf.byte + 0 + 1)

This patch is to skip some tests by checking proc valid_addr_p.
We can handle other targets in valid_addr_p too.

gdb/testsuite:

2015-03-16  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/watchpoint-reuse-slot.exp (valid_addr_p): New proc.
	(top level): Skip tests if valid_addr_p returns false for
	$cmd1 or $cmd2.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2015-03-16  Yao Qi  <yao.qi@linaro.org>
+
+	* gdb.base/watchpoint-reuse-slot.exp (valid_addr_p): New proc.
+	(top level): Skip tests if valid_addr_p returns false for
+	$cmd1 or $cmd2.
+
 2015-03-11  Andy Wingo  <wingo@igalia.com>
 
 	* gdb.guile/scm-objfile.exp: Add objfile-progspace test.

gdb/testsuite/gdb.base/watchpoint-reuse-slot.exp

@@ -133,6 +133,28 @@ delete_breakpoints
 
 set cur_addr [get_pc]
 
+# Return true if the memory range [buf.byte + OFFSET, +WIDTH] can be
+# monitored by CMD, otherwise return false.
+
+proc valid_addr_p {cmd offset width} {
+
+    if { [istarget "aarch64*-*-linux*"] } {
+	# The aarch64 Linux kernel port only accepts 4-byte aligned addresses
+	# for hardware breakpoints and 8-byte aligned addresses for hardware
+	# watchpoints.  However, both GDB and GDBserver support unaligned
+	# watchpoints by using more than one properly aligned watchpoint
+	# registers to represent the whole unaligned region.  Breakpoint
+	# addresses must still be aligned though.
+	if {$cmd == "hbreak" } {
+	    if { [expr ($offset) % 4] != 0 } {
+		return 0
+	    }
+	}
+    }
+
+    return 1
+}
+
 # Watch WIDTH bytes at BASE + OFFSET.  CMD specifices the specific
 # type of watchpoint to use.  If CMD is "hbreak", WIDTH is ignored.
 
@@ -172,6 +194,15 @@ foreach always_inserted {"off" "on" } {
 		}
 
 		for {set x 0} {$x < 4} {incr x} {
+
+		    if { ![valid_addr_p $cmd1 $x $width]
+			 || ![valid_addr_p $cmd2 $x+1 $width] } {
+			# Skip tests if requested address or length
+			# of breakpoint or watchpoint don't meet
+			# target or kernel requirements.
+			continue
+		    }
+
 		    set prefix "always-inserted $always_inserted: "
 		    append prefix "$cmd1 x $cmd2: "
 		    with_test_prefix "$prefix: width $width, iter $x" {