runtime: use the correct AT_HWCAP auxv constant on freebsd

In golang.org/cl/102355 I mistakenly used 26 instead of 25 as the AT_HWCAP value.
26 is AT_HWCAP2. While experimenting with FreeBSD-11.2-BETA3 (where both values are
being supplied in the auxv), the AT_HWCAP2 value read is 0 which triggers the error again:

  runtime: this CPU has no floating point hardware, so it cannot run this GOARM=7 binary. Recompile using GOARM=5.

Updates #24507.

Change-Id: Ide04b7365d8f10e4650edf4e188dd58bdf42cc26
Reviewed-on: https://go-review.googlesource.com/114822Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/os_freebsd.go

@@ -382,7 +382,7 @@ const (
 	_AT_NULL     = 0  // Terminates the vector
 	_AT_PAGESZ   = 6  // Page size in bytes
 	_AT_TIMEKEEP = 22 // Pointer to timehands.
-	_AT_HWCAP    = 26 // CPU feature flags
+	_AT_HWCAP    = 25 // CPU feature flags
 )
 
 func sysauxv(auxv []uintptr) {