<rdar://problem/6940611> libLTO.dylib needs to let linker specify path to assembler

Add lto_codegen_set_assembler_path() API which allows the linker to specify the
path to the assembler tool to run.  When assembler is used (instead of compiler)
different command line options are used.
Add LTO_API_VERSION #define so clients (linkers) can conditionalize use of new APIs.

llvm-svn: 72823