[CMake] Remove all uses of LLVM_NATIVE_ARCH.

Instead, assume we're going to target triple specified by
COMPILER_RT_DEFAULT_TARGET_TRIPLE and build runtimes for this triple
(and hope that the host compiler can target them).

This will help users that use cross-compiler on their host to build
Clang that would work on a different architecture. This will also come in
handy if one would want to configure several compiler-rt build trees on
the same host, using just-built Clang that can target many
architectures.

This doesn't change the behavior in the default build configuration.

llvm-svn: 247099