cmd/compiler,cmd/go,sync: add internal {LoadAcq,StoreRel}64 on ppc64

Add an internal atomic intrinsic for load with acquire semantics
(extending LoadAcq to 64b) and add LoadAcquintptr for internal
use within the sync package.  For other arches, this remaps to the
appropriate atomic.Load{,64} intrinsic which should not alter code
generation.

Similarly, add StoreRel{uintptr,64} for consistency, and inline.

Finally, add an exception to allow sync to directly use the
runtime/internal/atomic package which avoids more convoluted
workarounds (contributed by Lynn Boger).

In an extreme example, sync.(*Pool).pin consumes 20% of wall time
during fmt tests.  This is reduced to 5% on ppc64le/power9.

From the fmt benchmarks on ppc64le:

name                           old time/op  new time/op  delta
SprintfPadding                  468ns ± 0%   451ns ± 0%   -3.63%
SprintfEmpty                   73.3ns ± 0%  51.9ns ± 0%  -29.20%
SprintfString                   135ns ± 0%   122ns ± 0%   -9.63%
SprintfTruncateString           232ns ± 0%   214ns ± 0%   -7.76%
SprintfTruncateBytes            216ns ± 0%   202ns ± 0%   -6.48%
SprintfSlowParsingPath          162ns ± 0%   142ns ± 0%  -12.35%
SprintfQuoteString             1.00µs ± 0%  0.99µs ± 0%   -1.39%
SprintfInt                      117ns ± 0%   104ns ± 0%  -11.11%
SprintfIntInt                   190ns ± 0%   175ns ± 0%   -7.89%
SprintfPrefixedInt              232ns ± 0%   212ns ± 0%   -8.62%
SprintfFloat                    270ns ± 0%   255ns ± 0%   -5.56%
SprintfComplex                 1.01µs ± 0%  0.99µs ± 0%   -1.68%
SprintfBoolean                  127ns ± 0%   111ns ± 0%  -12.60%
SprintfHexString                220ns ± 0%   198ns ± 0%  -10.00%
SprintfHexBytes                 261ns ± 0%   252ns ± 0%   -3.45%
SprintfBytes                    600ns ± 0%   590ns ± 0%   -1.67%
SprintfStringer                 684ns ± 0%   658ns ± 0%   -3.80%
SprintfStructure               2.57µs ± 0%  2.57µs ± 0%   -0.12%
ManyArgs                        669ns ± 0%   646ns ± 0%   -3.44%
FprintInt                       140ns ± 0%   136ns ± 0%   -2.86%
FprintfBytes                    184ns ± 0%   181ns ± 0%   -1.63%
FprintIntNoAlloc                140ns ± 0%   136ns ± 0%   -2.86%
ScanInts                        929µs ± 0%   921µs ± 0%   -0.79%
ScanRecursiveInt                122ms ± 0%   121ms ± 0%   -0.11%
ScanRecursiveIntReaderWrapper   122ms ± 0%   122ms ± 0%   -0.18%

Change-Id: I4d66780261b57b06ef600229e475462e7313f0d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/253748


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>