cmd/asm: add PCALIGN support on 386/amd64
The PCALIGN asm directive was not supported on 386/amd64,
causing a compile-time error when used. The same directive
is currently supported on arm64, loong64 and ppc64 architectures.
This has potential for noticeable performance improvements on
amd64 across multiple packages, I did a quick test aligning a hot
loop on bytes.IndexByte:
```
IndexByte/10-16 3.477n ± ∞ ¹ 3.462n ± ∞ ¹ ~ (p=0.198 n=5)
IndexByte/32-16 4.675n ± ∞ ¹ 4.834n ± ∞ ¹ +3.40% (p=0.008 n=5)
IndexByte/4K-16 67.47n ± ∞ ¹ 44.44n ± ∞ ¹ -34.13% (p=0.008 n=5)
IndexByte/4M-16 61.98µ ± ∞ ¹ 45.07µ ± ∞ ¹ -27.28% (p=0.008 n=5)
IndexByte/64M-16 1206.6µ ± ∞ ¹ 940.9µ ± ∞ ¹ -22.02% (p=0.008 n=5)
IndexBytePortable/10-16 4.064n ± ∞ ¹ 4.044n ± ∞ ¹ ~ (p=0.325 n=5)
IndexBytePortable/32-16 9.999n ± ∞ ¹ 9.934n ± ∞ ¹ ~ (p=0.151 n=5)
IndexBytePortable/4K-16 975.8n ± ∞ ¹ 965.5n ± ∞ ¹ ~ (p=0.151 n=5)
IndexBytePortable/4M-16 973.3µ ± ∞ ¹ 972.3µ ± ∞ ¹ ~ (p=0.222 n=5)
IndexBytePortable/64M-16 15.68m ± ∞ ¹ 15.89m ± ∞ ¹ ~ (p=0.310 n=5)
geomean 1.478µ 1.342µ -9.20%
IndexByte/10-16 2.678Gi ± ∞ ¹ 2.690Gi ± ∞ ¹ ~ (p=0.151 n=5)
IndexByte/32-16 6.375Gi ± ∞ ¹ 6.165Gi ± ∞ ¹ -3.30% (p=0.008 n=5)
IndexByte/4K-16 56.54Gi ± ∞ ¹ 85.85Gi ± ∞ ¹ +51.83% (p=0.008 n=5)
IndexByte/4M-16 63.03Gi ± ∞ ¹ 86.68Gi ± ∞ ¹ +37.52% (p=0.008 n=5)
IndexByte/64M-16 51.80Gi ± ∞ ¹ 66.42Gi ± ∞ ¹ +28.23% (p=0.008 n=5)
IndexBytePortable/10-16 2.291Gi ± ∞ ¹ 2.303Gi ± ∞ ¹ ~ (p=0.421 n=5)
IndexBytePortable/32-16 2.980Gi ± ∞ ¹ 3.000Gi ± ∞ ¹ ~ (p=0.151 n=5)
IndexBytePortable/4K-16 3.909Gi ± ∞ ¹ 3.951Gi ± ∞ ¹ ~ (p=0.151 n=5)
IndexBytePortable/4M-16 4.013Gi ± ∞ ¹ 4.017Gi ± ∞ ¹ ~ (p=0.222 n=5)
IndexBytePortable/64M-16 3.987Gi ± ∞ ¹ 3.933Gi ± ∞ ¹ ~ (p=0.310 n=5)
geomean 8.183Gi 9.013Gi +10.14%
```
Fixes #56474
Change-Id: Idea022b1a16e6d4b8dd778723adb862c46602c4f
GitHub-Last-Rev:
2eb7e31dc378a02fd83faa7d41239df0f2859677
GitHub-Pull-Request: golang/go#61516
Reviewed-on: https://go-review.googlesource.com/c/go/+/511662
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>