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

crypto/asn1/a_time.c

@@ -597,11 +597,17 @@ int ASN1_TIME_compare(const ASN1_TIME *a, const ASN1_TIME *b)
 # define timezone _timezone
 #endif
 
+#ifdef __FreeBSD__
+# define USE_TIMEGM
+#endif
+
 time_t asn1_string_to_time_t(const char *asn1_string)
 {
     ASN1_TIME *timestamp_asn1 = NULL;
     struct tm *timestamp_tm = NULL;
+#ifndef USE_TIMEGM
     time_t timestamp_local;
+#endif
     time_t timestamp_utc;
 
     timestamp_asn1 = ASN1_TIME_new();
@@ -619,10 +625,15 @@ time_t asn1_string_to_time_t(const char *asn1_string)
         return -1;
     }
 
+#ifdef USE_TIMEGM
+    timestamp_utc = timegm(timestamp_tm);
+    OPENSSL_free(timestamp_tm);
+#else
     timestamp_local = mktime(timestamp_tm);
     OPENSSL_free(timestamp_tm);
 
     timestamp_utc = timestamp_local - timezone;
+#endif
 
     ASN1_TIME_free(timestamp_asn1);
     return timestamp_utc;