[libcxxabi][ARM] Make CXX_end_cleanup compatible with Armv6-M

On Armv6-M the branch may not able to reach the _Unwind_Resume function because it's relocation(R_ARM_THM_JUMP11) is in -2048, 2047 range only.

Reviewed By: chill, stuij, lenary

Differential Revision: https://reviews.llvm.org/D113181

libcxxabi/src/cxa_exception.cpp

@@ -382,7 +382,8 @@ asm("	.pushsection	.text.__cxa_end_cleanup,\"ax\",%progbits\n"
     "	bl	__cxa_end_cleanup_impl\n"
     "	pop	{r1, r2, r3, r4}\n"
     "	mov	lr, r4\n"
-    "	b	_Unwind_Resume\n"
+    "	ldr r4,	=_Unwind_Resume\n"
+    "	bx	r4\n"
     "	.popsection");
 #endif // defined(_LIBCXXABI_ARM_EHABI)