gn build: Rename llvm_host_triple to llvm_current_triple and have it use current_{cpu,os}.

This makes e.g. ToolChain::isCrossCompiling() in
clang/lib/Driver/ToolChain.cpp return the correct result
if the compiler was cross-compiled. This change also affects
llvm_default_target_triple, so cross-compiled compilers default to
targeting the cross-compilation target, which makes more sense than
the host that the compiler was compiled on.

This change will also be necessary in order for the correct triples
to appear in generated lit files for non-native targets.

Differential Revision: https://reviews.llvm.org/D56696

llvm-svn: 351168