Add missing __gnu_Unwind_Find_exidx symbol

Also since __aeabi* and __gnu_Unwind_Find_exidx symbols
were incorrectly versioned in M (android-23) remove them from
the version script for all platform versions <= 23

Bug: https://github.com/android-ndk/ndk/issues/1
Change-Id: I04902aac25a722100fc502600df908bb3a786356

ndk/platforms/android-21/arch-arm/symbols/libc.so.functions.txt

@@ -49,6 +49,7 @@ __getcpu
 __getcwd
 __getpid
 __getpriority
+__gnu_Unwind_Find_exidx
 __hostalias
 __ioctl
 __isfinite

ndk/platforms/android-21/arch-arm/symbols/libc.so.versions.txt

@@ -1,19 +1,6 @@
 # Generated by genversionscripts.py. Do not edit.
 LIBC {
   global:
-    __aeabi_atexit; # arm
-    __aeabi_memclr; # arm
-    __aeabi_memclr4; # arm
-    __aeabi_memclr8; # arm
-    __aeabi_memcpy; # arm
-    __aeabi_memcpy4; # arm
-    __aeabi_memcpy8; # arm
-    __aeabi_memmove; # arm
-    __aeabi_memmove4; # arm
-    __aeabi_memmove8; # arm
-    __aeabi_memset; # arm
-    __aeabi_memset4; # arm
-    __aeabi_memset8; # arm
     __assert;
     __assert2;
     __atomic_cmpxchg; # arm