The procyield() function should yield the processor as in other
architectures. On ppc64x, this is achieved by setting the Program
Priority Register to 'low priority' prior to the spin loop, and
setting it back to 'medium-low priority' afterwards.

benchmark                          old ns/op     new ns/op     delta
BenchmarkMakeChan/Byte-8           87.7          86.6          -1.25%
BenchmarkMakeChan/Int-8            107           106           -0.93%
BenchmarkMakeChan/Ptr-8            201           204           +1.49%
BenchmarkMakeChan/Struct/0-8       78.2          79.7          +1.92%
BenchmarkMakeChan/Struct/32-8      196           200           +2.04%
BenchmarkMakeChan/Struct/40-8      236           230           -2.54%
BenchmarkChanNonblocking-8         8.64          8.85          +2.43%
BenchmarkChanUncontended-8         5577          5598          +0.38%
BenchmarkChanContended-8           66106         51529         -22.05%
BenchmarkChanSync-8                451           441           -2.22%
BenchmarkChanSyncWork-8            9155          9170          +0.16%
BenchmarkChanProdCons0-8           1585          1083          -31.67%
BenchmarkChanProdCons10-8          1094          838           -23.40%
BenchmarkChanProdCons100-8         831           657           -20.94%
BenchmarkChanProdConsWork0-8       1471          941           -36.03%
BenchmarkChanProdConsWork10-8      1033          721           -30.20%
BenchmarkChanProdConsWork100-8     730           511           -30.00%
BenchmarkChanCreation-8            135           128           -5.19%
BenchmarkChanSem-8                 602           463           -23.09%
BenchmarkChanPopular-8             3017466       2188441       -27.47%

Fixes #25625

Change-Id: Iacb1c888d3c066902152b8367500348fb631c5f9
Reviewed-on: https://go-review.googlesource.com/115376
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>