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/114822


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>