support --target-arch wasm32 (#1094)

Add wasm32 support to the build-obj, build-exe and build-lib commands
of the stage 1 compiler.  Wasm64 should work transparently once it's
supported in upstream LLVM.

To export a function:

    // lib.zig - for exposition, not necessary for this example
    pub use @import("add.zig");

    // add.zig
    export fn add(a: i32, b: i32) i32 {
        return a + b;
    }

To import a function:

    // cube.zig
    extern fn square(x: i32) i32;

    export fn cube(x: i32) i32 {
        return x * square(x);
    }

src/target.cpp

@@ -597,12 +597,15 @@ void resolve_target_object_format(ZigTarget *target) {
         case ZigLLVM_tce:
         case ZigLLVM_tcele:
         case ZigLLVM_thumbeb:
-        case ZigLLVM_wasm32:
-        case ZigLLVM_wasm64:
         case ZigLLVM_xcore:
             target->oformat= ZigLLVM_ELF;
             return;
 
+        case ZigLLVM_wasm32:
+        case ZigLLVM_wasm64:
+            target->oformat = ZigLLVM_Wasm;
+            return;
+
         case ZigLLVM_ppc:
         case ZigLLVM_ppc64:
             if (is_os_darwin(target)) {