Change _LIBUNWIND_SUPPORT_ARM_EHABI to LIBCXX_ARM_EHABI.

The latter define is used most places already in our code base,
just switch the few places that use the first to the latter and
remove it.

Change-Id: Iedf31ff63bd3cea8c387abec80224bc9c1956388
Merge pull request #118 from awong-dev/define

Change _LIBUNWIND_SUPPORT_ARM_EHABI to LIBCXX_ARM_EHABI.