runtime: fix AT_HWCAP auxv parsing on freebsd

AT_HWCAP is not available on FreeBSD-11.1-RELEASE or earlier and the wrong const was used.
Use the correct value, and initialize hwcap with ^uint32(0) inorder not to fail the VFP tests.

Fixes #24507.

Change-Id: I5c3eed57bb53bf992b7de0eec88ea959806306b9
Reviewed-on: https://go-review.googlesource.com/102355Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/os_freebsd.go

@@ -380,7 +380,7 @@ func sysargs(argc int32, argv **byte) {
 const (
 	_AT_NULL   = 0  // Terminates the vector
 	_AT_PAGESZ = 6  // Page size in bytes
-	_AT_HWCAP  = 16 // CPU feature flags
+	_AT_HWCAP  = 26 // CPU feature flags
 )
 
 func sysauxv(auxv []uintptr) {

src/runtime/os_freebsd_arm.go

@@ -10,8 +10,8 @@ const (
 	_HWCAP_IDIVA = 1 << 17
 )
 
-var hwcap uint32 // set by archauxv
-var hardDiv bool // set if a hardware divider is available
+var hwcap = ^uint32(0) // set by archauxv
+var hardDiv bool       // set if a hardware divider is available
 
 func checkgoarm() {
 	if goarm > 5 && hwcap&_HWCAP_VFP == 0 {