add llvm configure flags to enable/disable target OSes/Envs

Add configure flags --enable-target-oses=os1,os2 and --enable-target-envs=env1,env2 similar to --enable-target=arch1,arch2. 

These flags generate pre-processor defines that are then checked within the Triple module to force certain code paths to be considered dead (if llvm is built with an LTO-enabled-compiler). 

The if-defs could be spread further to give the same benefit for non-LTO compilers, but that isn't done right now... 

Concretely, if you build LLVM w/ clang -O4 and --enable-target-oses=Linux, there is about a 170 KB savings (most of the MC COFF support, plus some branches in various places). If --enable-target-envs=gnu is also specified then most of MC MachO code is also pruned: 

lto, all oses, all environments: 12757900 
lto, only Linux, all environments: 12584978 
lto, only Linux, only gnu environment: 12528216