return zero in arm_linux_can_use_hw_breakpoint if HW point isn't supported

This patch is to cherry-pick part of Pedro's patch here
https://sourceware.org/ml/gdb-patches/2015-04/msg00527.html in which
zero is returned if the HW point isn't supported.

In arm-linux native gdb testing on a board doesn't support HW breakpoint,
without this patch, the output in gdb.base/breakpoint-in-ro-region.exp is like:

(gdb) hbreak *0x83bc^M
Hardware breakpoints used exceeds limit.^M
(gdb) PASS: gdb.base/breakpoint-in-ro-region.exp: probe hbreak support (support)

with this patch, the output becomes:

(gdb) hbreak *0x83bc^M
No hardware breakpoint support in the target.^M
(gdb) PASS: gdb.base/breakpoint-in-ro-region.exp: probe hbreak support (no support)

As a result, the following fails are fixed.

-FAIL: gdb.base/breakpoint-in-ro-region.exp: always-inserted off: auto-hw on: step in ro region (cannot insert hw break)
-FAIL: gdb.base/breakpoint-in-ro-region.exp: always-inserted off: auto-hw on: thread advanced
-FAIL: gdb.base/breakpoint-in-ro-region.exp: always-inserted on: auto-hw on: step in ro region (cannot insert hw break)
-FAIL: gdb.base/breakpoint-in-ro-region.exp: always-inserted on: auto-hw on: thread advanced

gdb:

2015-04-17  Pedro Alves  <palves@redhat.com>

	* arm-linux-nat.c (arm_linux_can_use_hw_breakpoint): Return zero
	if HW point of TYPE isn't supported.

1 files changed, 10 insertions, 2 deletions

diff --git a/gdb/arm-linux-nat.c b/gdb/arm-linux-nat.c
index bb8358c..afc5817 100644
--- a/gdb/arm-linux-nat.c
+++ b/gdb/arm-linux-nat.c
@@ -771,12 +771,20 @@ arm_linux_can_use_hw_breakpoint (struct target_ops *self,
   if (type == bp_hardware_watchpoint || type == bp_read_watchpoint
       || type == bp_access_watchpoint || type == bp_watchpoint)
     {
-      if (cnt + ot > arm_linux_get_hw_watchpoint_count ())
+      int count = arm_linux_get_hw_watchpoint_count ();
+
+      if (count == 0)
+	return 0;
+      else if (cnt + ot > count)
 	return -1;
     }
   else if (type == bp_hardware_breakpoint)
     {
-      if (cnt > arm_linux_get_hw_breakpoint_count ())
+      int count = arm_linux_get_hw_breakpoint_count ();
+
+      if (count == 0)
+	return 0;
+      else if (cnt > count)
 	return -1;
     }
   else