Fix backtraces on ARM EHABI.

Before this patch, our rust_eh_personality_catch routine would cut
backtracing short at the __rust_try function, due to it not handling
the _US_FORCE_UNWIND bit properly, which is passed by libunwind
implementations on ARM EHABI.

Examples of where the _US_FORCE_UNWIND bit is passed to the PR:
- GCC's libunwind: f1717362de/libgcc/unwind-arm-common.inc (L590)
- LLVM's libunwind: 61278584b5/src/UnwindLevel1-gcc-ext.c (L153)

src/libstd/sys/common/unwind/gcc.rs

@@ -224,8 +224,13 @@ pub mod eabi {
         context: *mut uw::_Unwind_Context
     ) -> uw::_Unwind_Reason_Code
     {
+        // Backtraces on ARM will call the personality routine with
+        // state == _US_VIRTUAL_UNWIND_FRAME | _US_FORCE_UNWIND. In those cases
+        // we want to continue unwinding the stack, otherwise all our backtraces
+        // would end at __rust_try.
         if (state as c_int & uw::_US_ACTION_MASK as c_int)
-                           == uw::_US_VIRTUAL_UNWIND_FRAME as c_int { // search phase
+                           == uw::_US_VIRTUAL_UNWIND_FRAME as c_int
+               && (state as c_int & uw::_US_FORCE_UNWIND as c_int) == 0 { // search phase
             uw::_URC_HANDLER_FOUND // catch!
         }
         else { // cleanup phase