Migrate from __FreeBSD__ to __FreeBSD_version (aws#1562)

### Issues:
Resolves aws#1557

### Description of changes: 

[`__FreeBSD__`](https://docs.freebsd.org/en/books/porters-handbook/porting-dads/#porting-versions)
doesn't seem to be as well supported (or as accurate) as
`__FreeBSD_version`. This change migrates to
[`__FreeBSD_version`](https://docs.freebsd.org/en/books/porters-handbook/versions/#versions)
which appears to be the recommended way to check for versions.

### Call-outs:
`__FreeBSD_version` is defined in
[`sys/param.h`](https://cgit.freebsd.org/src/tree/sys/sys/param.h#n76)

### Testing:
We don't have FreeBSD in our CI, a similar but not exactly the same
change was tested by the requester in
aws#1557.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

crypto/fipsmodule/rand/urandom.c

@@ -71,7 +71,8 @@
 
 #if defined(OPENSSL_FREEBSD)
 #define URANDOM_BLOCKS_FOR_ENTROPY
-#if __FreeBSD__ >= 12
+#include <sys/param.h>
+#if __FreeBSD_version >= 1200000
 // getrandom is supported in FreeBSD 12 and up.
 #define FREEBSD_GETRANDOM
 #include <sys/random.h>