[arm] watchpoint-reuse-slot.exp: skip setting HW points on some address

Hi,
ARM linux kernel has some requirements on the address/length setting
for HW breakpoints/watchpoints, but watchpoint-reuse-slot.exp doesn't
consider them and sets HW points on various addresses.  Many fails
are causes as a result:

stepi^M
Warning:^M
Could not insert hardware watchpoint 20.^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 watch: : width 2, iter 2: base + 1: stepi advanced

watch *(buf.byte + 2 + 1)@2^M
Hardware watchpoint 388: *(buf.byte + 2 + 1)@2^M
Warning:^M
Could not insert hardware watchpoint 388.^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 watch: : width 2, iter 2: base + 1: watch *(buf.byte + 2 + 1)@2

This patch is to reflect kernel requirements in watchpoint-reuse-slot.exp
in order to skip some tests.

gdb/testsuite:

2015-04-10  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/watchpoint-reuse-slot.exp (valid_addr_p): Return
	false for some offset and width combinations which aren't
	supported by linux kernel.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2015-04-10  Yao Qi  <yao.qi@linaro.org>
+
+	* gdb.base/watchpoint-reuse-slot.exp (valid_addr_p): Return
+	false for some offset and width combinations which aren't
+	supported by linux kernel.
+
 2015-04-10  Pedro Alves  <palves@redhat.com>
 
 	* gdb.threads/step-over-lands-on-breakpoint.exp (do_test): New

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

@@ -150,6 +150,33 @@ proc valid_addr_p {cmd offset width} {
 		return 0
 	    }
 	}
+    } elseif { [istarget "arm*-*-linux*"] } {
+	if { $cmd == "hbreak" } {
+	    # Breakpoints must be of length 2 (thumb) or 4 (ARM) bytes.
+	    if { $width != 2 && $width != 4 } {
+		return 0
+	    }
+	} else {
+	    # Watchpoints can be of length 1, 2, 4 or 8 bytes.
+	    if { [expr $width % 2] != 0 } {
+		return 0
+	    }
+	}
+
+	if { [expr ($offset) % 8] == 0 && $width == 8 } {
+	    # If WIDTH is 8 byte, the address should be 8-byte aligned.
+	    return 1
+	} elseif { [expr ($offset) % 4] == 0 } {
+	    return 1
+	} elseif { [expr ($offset) % 4] == 2 && $width == 2 } {
+	    # Halfword watchpoints and breakpoints.
+	    return 1
+	} elseif { [expr ($offset) % 4] == 1 && $width == 1 && $cmd != "hbreak" } {
+	    # Single byte watchpoints.
+	    return 1
+	} else {
+	    return 0
+	}
     }
 
     return 1