gate avx512 behind go1.22

PCALIGN gives a ~12% performance improvement so we really want it, if you are using a previous version of go, too bad please upgrade.

xxhash_asm_amd64.go

@@ -1,5 +1,5 @@
-//go:build amd64 && !appengine && gc && !purego
-// +build amd64,!appengine,gc,!purego
+//go:build amd64 && !appengine && gc && !purego && go1.22
+// +build amd64,!appengine,gc,!purego,go1.22
 
 package xxhash
 

xxhash_asm_amd64_old.go

@@ -0,0 +1,22 @@
+//go:build amd64 && !appengine && gc && !purego && !go1.22
+// +build amd64,!appengine,gc,!purego,!go1.22
+
+// The avx512 impl relies on PCALIGN.
+
+package xxhash
+
+// Sum64 computes the 64-bit xxHash digest of b with a zero seed.
+func Sum64(b []byte) uint64 {
+	return sum64Scalar(b)
+}
+
+//go:noescape
+func sum64Scalar(b []byte) uint64
+
+// extra is a first block before b, it may be nil then skip it.
+func writeBlocks(d *Digest, extra *[32]byte, b []byte) {
+	return writeBlocksScalar(d, extra, b)
+}
+
+//go:noescape
+func writeBlocksScalar(d *Digest, extra *[32]byte, b []byte)

xxhash_avx512_amd64.s

@@ -1,7 +1,8 @@
-//go:build !appengine && gc && !purego
+//go:build !appengine && gc && !purego && go1.22
 // +build !appengine
 // +build gc
 // +build !purego
+// +build go1.22
 
 #include "textflag.h"