[libcxxabi/runtimes] Set LLVM_HOST_TRIPLE in runtimes build

This allows tests to tell if they're running natively.

Those tests are libcxxabi/test/native/arm-linux-eabi.
Which were running on Linaro's bots but became unsupported
when we switched to the runtimes build.

Reviewed By: #libc_abi, phosek

Differential Revision: https://reviews.llvm.org/D113663