runtime: fix setting of cpu features for amd64

Because of wrong case of letters, the cpu features flags were not
set properly for amd64.

Fixes #48406.

Change-Id: If19782851670e91fd31d119f4701c47373fa7e71
GitHub-Last-Rev: 91c7321
GitHub-Pull-Request: #48403
Reviewed-on: https://go-review.googlesource.com/c/go/+/350151
Trust: Keith Randall <[email protected]>
Reviewed-by: Austin Clements <[email protected]>
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: nimelehin

src/runtime/proc.go

@@ -623,7 +623,7 @@ func cpuinit() {
 	// Support cpu feature variables are used in code generated by the compiler
 	// to guard execution of instructions that can not be assumed to be always supported.
 	switch GOARCH {
-	case "386", "AMD64":
+	case "386", "amd64":
 		x86HasPOPCNT = cpu.X86.HasPOPCNT
 		x86HasSSE41 = cpu.X86.HasSSE41
 		x86HasFMA = cpu.X86.HasFMA