arm: detect NEON on FreeBSD armv6

FreeBSD 11.2 added AT_HWCAP via sysctl() while 12.0 added elf_aux_info()
which requires less code to use and is cached by libc. Only support the
latter to avoid cruft to be removed after 11.* EOL on 2021-09-30.
14 jobs for master in 6 minutes and 8 seconds (queued for 13 seconds)
Status Job ID Name Coverage
  Style
passed #245021
amd64 debian
style-check

00:00:20

 
  Build
passed #245022
amd64 debian
build-debian

00:00:35

passed #245027
debian aarch64
build-debian-aarch64

00:01:26

passed #245028
debian aarch64
build-debian-aarch64-clang-5

00:01:10

passed #245023
amd64 debian
build-debian-static

00:00:34

passed #245030
debian aarch64
build-debian-werror

00:00:32

passed #245024
amd64 debian
build-debian32

00:00:27

passed #245029
macos
build-macos

00:00:26

passed #245025
win32
build-win32

00:00:28

passed #245026
win64
build-win64

00:00:36

 
  Test
passed #245031
amd64 debian
test-debian

00:00:45

passed #245032
amd64 debian
test-debian-asan

00:02:09

passed #245033
amd64 debian
test-debian-msan

00:00:58

passed #245034
amd64 debian
test-debian-ubsan

00:01:20