security/putty: fix aarch64 build on clang 10.

clang 10 (FreeBSD 12.3) does not support vaddq_p128 and similar
ARM NEON intrinsics, but clang 13 (FreeBSD 13.1) does.

Abuse the HAVE_NEON_PMULL cmake test to disable aesgcm-neon.c
unless vaddq_p128() is available.

http://ampere1.nyi.freebsd.org/data/123arm64-quarterly/31229ca1324c/logs/putty-nogtk-0.78~pre20220922.log:
/wrkdirs/usr/ports/security/putty-nogtk/work/putty-0.78~pre20220922.9fcfd67/crypto/aesgcm-neon.c:125:16:
warning: implicit declaration of function 'vaddq_p128' is invalid in C99 [-Wimplicit-function-declaration]
ctx->acc = vaddq_p128(ctx->acc, load_p128_be(coeff));