[Headers][ARM] Allow `struct _Unwind_Exception` in unwind.h

Use the same approach as used in libunwind/include/unwind_arm_ehabi.h
(D89570) and GCC's unwind-arm-common.h, so that _Unwind_Exception can be
used both with and without the struct tag.

Fixes a build failure in libcxxabi/test/forced_unwind1.pass.cpp.

Reviewed By: danielkiss

Differential Revision: https://reviews.llvm.org/D139503