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

This reverts commit 3255578.

Failed buildbot's Armv7 builds:
https://lab.llvm.org/buildbot/#/builders/60/builds/5303

Author: vvereschaka

libcxxabi/src/cxa_exception.cpp

@@ -382,8 +382,7 @@ 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"
-    "	ldr r4,	=_Unwind_Resume\n"
-    "	bx	r4\n"
+    "	b	_Unwind_Resume\n"
     "	.popsection");
 #endif // defined(_LIBCXXABI_ARM_EHABI)