Provide temporary fallback for Snotime via time.Now() on linux/amd64/go1.17 due to Go's assembly implementation being removed on this target.

This is a (hopefully temporary) performance regression on this target: the call involves getting the monotonic clock time and back-and-forth conversions while we only need the wall clock time, which is what runtime.walltime1 used to provide. However, the removal from the runtime package necessitates backporting a significant portion of Go's runtime assembly for this target, where the previous bypass to achieve the perf boost over time.Now() was trivial in comparison, so I decided against maintaining such backport and will look into a more manageable solution instead.

Author: alcore

internal/time.go

@@ -1,4 +1,6 @@
-// +build !windows,!amd64
+//go:build !(windows && amd64) && !(linux && amd64 && go1.17)
+// +build !windows !amd64
+// +build !linux !amd64 !go1.17
 
 package internal
 

internal/time_linux_amd64.go

@@ -0,0 +1,11 @@
+//go:build go1.17
+// +build go1.17
+
+package internal
+
+import "time"
+
+// Snotime returns the current wall clock time reported by the OS as adjusted to our internal epoch.
+func Snotime() uint64 {
+	return uint64(time.Now().UnixNano()-epochNsec) / timeUnit
+}