Disable backtrace on not supported uclibc.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>

diff -purN mono-native-3.8.0.orig/libgc/include/gc.h mono-native-3.8.0/libgc/include/gc.h
--- mono-native-3.8.0.orig/libgc/include/gc.h	2014-10-07 15:00:21.259466731 +0200
+++ mono-native-3.8.0/libgc/include/gc.h	2014-10-07 15:05:25.560975681 +0200
@@ -500,7 +500,7 @@ GC_API GC_PTR GC_malloc_atomic_ignore_of
 #ifdef __linux__
 # include <features.h>
 # if (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 1 || __GLIBC__ > 2) \
-     && !defined(__ia64__)
+     && !defined(__ia64__) && !defined(__UCLIBC__)
 #   ifndef GC_HAVE_BUILTIN_BACKTRACE
 #     define GC_HAVE_BUILTIN_BACKTRACE
 #   endif