cmd/internal/obj/wasm,cmd/link/internal/wasm: add fast path for writeUleb128

While building a simple hello world binary, there are total 858277 calls
to writeUleb during the assembler phase out of which 836625 (97%) are less than 7 bits.

Using a simple micro-benchmark like this:

func BenchmarkUleb(b *testing.B) {
	var buf bytes.Buffer
	for i := 0; i < b.N; i++ {
		writeUleb128(&buf, 42)
		buf.Reset()
	}
}

We get the following results with the fast path enabled.

name    old time/op  new time/op  delta
Uleb-4  8.45ns ± 2%  7.51ns ± 2%  -11.16%  (p=0.000 n=10+10)

Applying the time taken to the number of calls, we get roughly 6% improvement
in total time taken for writeUleb128.

We also apply the change to the function in linker to make it consistent.

Change-Id: I9fe8c41df1209f5f3aa7d8bd0181f1b0e536ceb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/201177
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/wasm/wasmobj.go

@@ -1154,6 +1154,10 @@ func align(as obj.As) uint64 {
 }
 
 func writeUleb128(w io.ByteWriter, v uint64) {
+	if v < 128 {
+		w.WriteByte(uint8(v))
+		return
+	}
 	more := true
 	for more {
 		c := uint8(v & 0x7f)

src/cmd/link/internal/wasm/asm.go

@@ -542,6 +542,10 @@ func writeName(w nameWriter, name string) {
 }
 
 func writeUleb128(w io.ByteWriter, v uint64) {
+	if v < 128 {
+		w.WriteByte(uint8(v))
+		return
+	}
 	more := true
 	for more {
 		c := uint8(v & 0x7f)