all: add support for more architectures and GOOS/GOARCH (#118)

This commit does two things:

  * It adds support for the GOOS and GOARCH environment variables. They
    fall back to runtime.GO* only when not available.
  * It adds support for 3 new architectures: 386, arm, and arm64. For
    now, this is Linux-only.

src/runtime/arch_386.go

@@ -0,0 +1,11 @@
+package runtime
+
+const GOARCH = "386"
+
+// The bitness of the CPU (e.g. 8, 32, 64).
+const TargetBits = 32
+
+// Align on word boundary.
+func align(ptr uintptr) uintptr {
+	return (ptr + 3) &^ 3
+}

src/runtime/arch_amd64.go

@@ -1,5 +1,3 @@
-// +build amd64
-
 package runtime
 
 const GOARCH = "amd64"

src/runtime/arch_arm.go

@@ -0,0 +1,11 @@
+package runtime
+
+const GOARCH = "arm"
+
+// The bitness of the CPU (e.g. 8, 32, 64).
+const TargetBits = 32
+
+// Align on word boundary.
+func align(ptr uintptr) uintptr {
+	return (ptr + 3) &^ 3
+}

src/runtime/arch_arm64.go

@@ -0,0 +1,11 @@
+package runtime
+
+const GOARCH = "arm64"
+
+// The bitness of the CPU (e.g. 8, 32, 64).
+const TargetBits = 64
+
+// Align on word boundary.
+func align(ptr uintptr) uintptr {
+	return (ptr + 7) &^ 7
+}