Add feature to determine if host architecture is 64-bit in llvm-lit

I have a test that I'd like to add to llvm that demands using more than
32-bits worth of address space. This test can't be run on 32-bit systems
because they don't have enough address space. The host triple should be
used to determine this instead of config.host_arch because on Debian
systems config.host_arch is not correct. This change adds the
"host-arch-is-64bit" feature to allow tests to restrict themselves to
the 64-bit case.

Differential Revision: https://reviews.llvm.org/D39465

llvm-svn: 317281