arm: libgcc_s: Fix ABI breakage introduced in r354347

Provide the symbol version for llvm-libunwind's _Unwind_Backtrace that libgcc
has historically provided on arm, in addition to the (default) standard version
used on all other arch.

Reported by:	mmel

contrib/libunwind/src/UnwindLevel1-gcc-ext.c

@@ -181,6 +181,10 @@ _Unwind_Backtrace(_Unwind_Trace_Fn callback, void *ref) {
     }
   }
 }
+#ifdef __arm__
+/* Preserve legacy libgcc ARM ABI mistake. */
+__sym_compat(_Unwind_Backtrace, _Unwind_Backtrace, GCC_4.3.0);
+#endif
 
 
 /// Find DWARF unwind info for an address 'pc' in some function.