[ANDROID] Typo in armv7-a clang toolchain

When toolchains upgraded to match Android NDK r12
format (clang version removed from toolchain suffix), a
small typo for the armeabi-v7a target was resulting into
wrongly choosing the GCC 4.9 toolchain when CLANG
flag was enabled.

Signed-off-by: Anestis Bechtsoudis <anestis@census-labs.com>

Makefile

diff --git a/Makefile b/Makefile
index 1f49952..4b75701 100644
--- a/Makefile
+++ b/Makefile
@@ -154,7 +154,7 @@
   # clang works only for APIs >= 23, so default to it if not set
   ANDROID_API ?= android-24
   ifeq ($(ANDROID_APP_ABI),armeabi-v7a)
-    ANDROID_NDK_TOOLCHAIN ?= arm-linux-androideabi-4.9
+    ANDROID_NDK_TOOLCHAIN ?= arm-linux-androideabi-clang
   else ifeq ($(ANDROID_APP_ABI),x86)
     ANDROID_NDK_TOOLCHAIN ?= x86-clang
   else ifeq ($(ANDROID_APP_ABI),arm64-v8a)