asn1_string_to_time_t: Use timegm on FreeBSD.

asn1_string_to_time_t: Use timegm on FreeBSD.

FreeBSD does not provide a global timezone variable containing the
offset to UTC.  Instead, FreeBSD's libc includes a legacy timezone
function dating back to Version 7 AT&T UNIX.  As a result,
asn1_string_to_time_t currently fails to compile on FreeBSD as it
subtracts a function from a time_t value:

../crypto/asn1/a_time.c:625:37: error: invalid operands to binary expression ('time_t' (aka 'long') and 'char *(int, int)')
    timestamp_utc = timestamp_local - timezone;
                    ~~~~~~~~~~~~~~~ ^ ~~~~~~~~
1 error generated.

However, FreeBSD's libc does include a non-standard (but widely
available) timegm function which converts a struct tm directly to a
UTC time_t value.  Use this on FreeBSD instead of mktime.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17765)