libcutils: Reflect the naming change, __ARM_HAVE_LDREX_STREX, in bionic

This patch reflect the changes that macro  _ARM_HAVE_LDREX_STREX had
been renamed to __ARM_HAVE_LDREX_STREX in bionic.

Change-Id: I03e8b9f95bdf6ed0da1f53326170f6ee57a37066
diff --git a/libcutils/atomic-android-arm.S b/libcutils/atomic-android-arm.S
index 1dd2363..4d98451 100644
--- a/libcutils/atomic-android-arm.S
+++ b/libcutils/atomic-android-arm.S
@@ -237,7 +237,7 @@
 
 /* replaced swp instruction with ldrex/strex for ARMv6 & ARMv7 */
 android_atomic_swap:
-#if defined (_ARM_HAVE_LDREX_STREX)
+#if defined (__ARM_HAVE_LDREX_STREX)
 1:  ldrex   r2, [r1]
     strex   r3, r0, [r1]
     teq     r3, #0