runtime: bump MaxGcprocs to 32

runtime: bump MaxGcprocs to 32
There was a number of improvements related to GC parallelization:
1. Parallel roots/stacks scanning.
2. Parallel stack shrinking.
3. Per-thread workbuf caches.
4. Workset reduction.
Currently 32 threads work well.
go.benchmarks:garbage benchmark on 2 x Intel Xeon E5-2690 (16 HT cores)

1 thread/1 processor:
time=16405255
cputime=16386223
gc-pause-one=546793975
gc-pause-total=3280763

2 threads/1 processor:
time=9043497
cputime=18075822
gc-pause-one=331116489
gc-pause-total=2152257

4 threads/1 processor:
time=4882030
cputime=19421337
gc-pause-one=174543105
gc-pause-total=1134530

8 threads/1 processor:
time=4134757
cputime=20097075
gc-pause-one=158680588
gc-pause-total=1015555

16 threads/1 processor + HT:
time=2006706
cputime=31960509
gc-pause-one=75425744
gc-pause-total=460097

16 threads/2 processors:
time=1513373
cputime=23805571
gc-pause-one=56630946
gc-pause-total=345448

32 threads/2 processors + HT:
time=1199312
cputime=37592764
gc-pause-one=48945064
gc-pause-total=278986

LGTM=rlh
R=golang-codereviews, tracey.brendan, rlh
CC=golang-codereviews, khr, rsc
https://golang.org/cl/123920043