runtime: document GODEBUG=cpu.* in runtime package

So far was only documented in doc/diagnostics.

Fixes: #54183

Change-Id: Iab67a1500547f30325453c614a2f2f6621dfadb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/423275
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
Run-TryBot: Michael Knyszek <[email protected]>
Reviewed-by: Martin Möhrmann <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Lynn Boger <[email protected]>

Author: Martin Möhrmann

src/runtime/extern.go

@@ -42,6 +42,12 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	clobber the memory content of an object with bad content when it frees
 	the object.
 
+	cpu.*: cpu.all=off disables the use of all optional instruction set extensions.
+	cpu.extension=off disables use of instructions from the specified instruction set extension.
+	extension is the lower case name for the instruction set extension such as sse41 or avx
+	as listed in internal/cpu package. As an example cpu.avx=off disables runtime detection
+	and thereby use of AVX instructions.
+
 	cgocheck: setting cgocheck=0 disables all checks for packages
 	using cgo to incorrectly pass Go pointers to non-Go code.
 	Setting cgocheck=1 (the default) enables relatively cheap