[libcxx] Set ABI version 2 as default for Fuchsia

This avoids the need for a custom generated config file which is desired
because the custom config files differs per-target which means we cannot
reuse headers across different targets.

Differential Revision: https://reviews.llvm.org/D45304

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@329770 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -36,7 +36,11 @@
 #define _LIBCPP_VERSION 7000
 
 #ifndef _LIBCPP_ABI_VERSION
-#define _LIBCPP_ABI_VERSION 1
+#  ifdef __Fuchsia__
+#    define _LIBCPP_ABI_VERSION 2
+#  else
+#    define _LIBCPP_ABI_VERSION 1
+#  endif
 #endif
 
 #if defined(__ELF__)