rustc: Add obj_is_bitcode to TargetOptions

This tells trans:🔙:write not to LLVM codegen to create .o
files but to put LLMV bitcode in .o files.

Emscripten's emcc supports .o in this format, and this is,
I think, slightly easier than making rlibs work without .o
files.

src/librustc_back/target/asmjs_unknown_emscripten.rs

@@ -22,6 +22,7 @@ pub fn target() -> Target {
         linker_is_gnu: true,
         allow_asm: false,
         archive_format: "gnu".to_string(),
+        obj_is_bitcode: true,
         .. Default::default()
     };
     Target {

src/librustc_back/target/mod.rs

@@ -204,6 +204,10 @@ pub struct TargetOptions {
     /// Flag indicating whether ELF TLS (e.g. #[thread_local]) is available for
     /// this target.
     pub has_elf_tls: bool,
+    // This is mainly for easy compatibility with emscripten.
+    // If we give emcc .o files that are actually .bc files it
+    // will 'just work'.
+    pub obj_is_bitcode: bool,
 }
 
 impl Default for TargetOptions {
@@ -251,6 +255,7 @@ impl Default for TargetOptions {
             exe_allocation_crate: "alloc_system".to_string(),
             allow_asm: true,
             has_elf_tls: false,
+            obj_is_bitcode: false,
         }
     }
 }