encoding/xml: parallelize benchmarks

encoding/xml: parallelize benchmarks

Results remain comparable with the non-parallel version with -cpu=1:
benchmark                old ns/op     new ns/op     delta
BenchmarkMarshal         31220         28618         -8.33%
BenchmarkMarshal-6       37181         7658          -79.40%
BenchmarkUnmarshal       81837         83522         +2.06%
BenchmarkUnmarshal-6     96339         18244         -81.06%

benchmark                old allocs     new allocs     delta
BenchmarkMarshal         23             23             +0.00%
BenchmarkMarshal-6       23             23             +0.00%
BenchmarkUnmarshal       189            189            +0.00%
BenchmarkUnmarshal-6     189            189            +0.00%

benchmark                old bytes     new bytes     delta
BenchmarkMarshal         5776          5776          +0.00%
BenchmarkMarshal-6       5776          5776          +0.00%
BenchmarkUnmarshal       8576          8576          +0.00%
BenchmarkUnmarshal-6     8576          8576          +0.00%

updates #18177

Change-Id: I7e7055a11d18896bd54d7d773f2ec64767cdb4c8
Reviewed-on: https://go-review.googlesource.com/36810
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>