Begin to create target definition system of our own.

Inspiration taken heavily from Rust.

src/compiler/crystal/codegen/target.cr

@@ -0,0 +1,19 @@
+abstract struct Crystal::Target
+  @triple: String?
+  @endian: String?
+  @pointer_width: String?
+  @os: String?
+  @env: String?
+  @vendor: String?
+  @arch: String?
+  @data_layout: String?
+  @option: Options?
+end
+
+abstract struct Crystal::Target::Options
+  @linker: String = ENV["CC"]? || "cc"
+  @ar: String = ENV["AR"]? || "ar"
+  @dynamic_linking: Bool = false
+  @executables: Bool = false
+  @executable_suffix: String = ""
+end

src/compiler/crystal/codegen/target/asmjs_unknown_emscripten.cr

@@ -0,0 +1,23 @@
+require "../target"
+
+module Crystal
+  struct Target::ASMJS::Emscripten < Target
+    @triple = "asmjs-unknown-target"
+    @endian = "little"
+    @pointer_width = "32"
+    @os = "emscripten"
+    @env = ""
+    @vendor = "unknown"
+    @data_layout = "e-p:32:32-i64:64-v128:32:128-n32-S128"
+    @arch = "asmjs"
+    @option = Options.new
+
+    struct Options < Target::Options
+      @linker = "emcc"
+      @ar = "emar"
+      @dynamic_linking = false
+      @executables = true
+      @executable_suffix = ".js"
+    end
+  end
+end

src/compiler/crystal/codegen/target/x86_64_apple_darwin.cr

@@ -0,0 +1,15 @@
+require "../target"
+
+
+module Crystal
+  struct Target::X86_64::Apple::Darwin < Target
+    triple = "x86_64-apple-darwin"
+    endian = "little"
+    pointer_width = "64"
+    data_layout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+    arch = "x86_64"
+    os = "macos"
+    env = ""
+    vendor = "apple"
+  end
+end