cmd/link: use BestSpeed for DWARF compression

DWARF compression accounts for roughly 30% of the linker's time. This
CL switches from DefaultCompression to BestSpeed, which virtually
eliminates this time. This roughly halves the overhead of handling
DWARF in the linker:

name \ time/op         nodwarf     dwarf        dwarf-speed
BuildCmdGoAll          10.0s ±11%   10.6s ± 5%   10.8s ± 5%
             nodwarf                  +6.41%       +8.03%
               dwarf                                 ~
LinkCmdGo              626ms ± 5%  1096ms ± 2%   860ms ± 2%
             nodwarf                 +75.17%      +37.36%
               dwarf                              -21.59%

Previously, enabling DWARF had a 75% overhead in link time for cmd/go.
This change reduces this overhead to 37% (a 22% reduction).

The effect on binary size is minimal compared to DefaultCompression,
and still substantially better than no compression:

             cmd/go bytes
nodwarf      10106953
dwarf        12159049 nodwarf+20%
dwarf-speed  12408905 nodwarf+23%
dwarf-nozlib 17766473 nodwarf+76%

Updates #26318.

Change-Id: I33bb7caa038a2753c29104501663daf4839e7054
Reviewed-on: https://go-review.googlesource.com/123356
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/link/internal/ld/data.go

@@ -2177,7 +2177,14 @@ func compressSyms(ctxt *Link, syms []*sym.Symbol) []byte {
 	binary.BigEndian.PutUint64(sizeBytes[:], uint64(total))
 	buf.Write(sizeBytes[:])
 
-	z := zlib.NewWriter(&buf)
+	// Using zlib.BestSpeed achieves very nearly the same
+	// compression levels of zlib.DefaultCompression, but takes
+	// substantially less time. This is important because DWARF
+	// compression can be a significant fraction of link time.
+	z, err := zlib.NewWriterLevel(&buf, zlib.BestSpeed)
+	if err != nil {
+		log.Fatalf("NewWriterLevel failed: %s", err)
+	}
 	for _, sym := range syms {
 		if _, err := z.Write(sym.P); err != nil {
 			log.Fatalf("compression failed: %s", err)