security/libgcrypt: Remove AVX512 support for FreeBSD 14 and 15

LLVM19 as shipped with FreeBSD 14 and 15 is incompete. Disable it by
default. Users who want AVX512 on FreeBSD < 16 must install LLVM20 or
newer and select the AVX512 option.

FreeBSD 16-CURRENT has LLVM21, with complete support for AVX512. Setting
the AVX512 option on 16-CURRENT is a NOP.

PR: 297987