[libcxxabi] Check __SEH__, when checking if ARM EHABI is implied

ARM EHABI isn't signalled by any specific compiler builtin define,
but is implied by the lack of defines specifying any other
exception handling mechanism, `__USING_SJLJ_EXCEPTIONS__` or
`__ARM_DWARF_EH__`.

As Windows SEH also can be used for unwinding, check for the
`__SEH__` define too, in the same way.

Differential Revision: https://reviews.llvm.org/D126866