cmd/asm,cmd/internal/obj/riscv: provide branch pseudo-instructions

Implement various branch pseudo-instructions for riscv64. These make it easier
to read/write assembly and will also make it easier for the compiler to generate
optimised code.

Change-Id: Ic31a7748c0e1495522ebecf34b440842b8d12c04
Reviewed-on: https://go-review.googlesource.com/c/go/+/226397
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/riscv/asm_test.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"testing"
 )
 
@@ -131,3 +132,20 @@ TEXT _stub(SB),$0-0
 		t.Errorf("%v\n%s", err, out)
 	}
 }
+
+func TestBranch(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping in short mode")
+	}
+	if runtime.GOARCH != "riscv64" {
+		t.Skip("Requires riscv64 to run")
+	}
+
+	testenv.MustHaveGoBuild(t)
+
+	cmd := exec.Command(testenv.GoToolPath(t), "test")
+	cmd.Dir = "testdata/testbranch"
+	if out, err := testenv.CleanCmdEnv(cmd).CombinedOutput(); err != nil {
+		t.Errorf("Branch test failed: %v\n%s", err, out)
+	}
+}