Windows 7 support (#2940)

### Issues:
Addresses #1997

### Description of changes: 
`ProcessPrng` (from `bcryptprimitives.dll`) is only available on Windows
8+. So the current code fails at runtime on Windows 7 because
`GetProcAddress` returns `NULL` and causes an `abort()`.

This change defines `AWSLC_WINDOWS_7_COMPAT` when `_WIN32_WINNT <=
_WIN32_WINNT_WIN7`. Then uses the existing `BCryptGenRandom` logic when
it's is defined.

* Related: aws/aws-lc-rs#1000

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.

Author: justsmth

crypto/rand_extra/windows.c

@@ -26,16 +26,23 @@ OPENSSL_MSVC_PRAGMA(warning(push, 3))
 
 #include <windows.h>
 
-#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP) && \
-    !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+// ProcessPrng (from `bcryptprimitives.dll`) is only available on Windows 8+.
+#if !defined(__MINGW32__) && defined(_WIN32_WINNT) && _WIN32_WINNT <= _WIN32_WINNT_WIN7
+#define AWSLC_WINDOWS_7_COMPAT
+#endif
+
+#if defined(AWSLC_WINDOWS_7_COMPAT) || \
+    (WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP) && \
+    !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP))
 #include <bcrypt.h>
 OPENSSL_MSVC_PRAGMA(comment(lib, "bcrypt.lib"))
-#endif  // WINAPI_PARTITION_APP && !WINAPI_PARTITION_DESKTOP
+#endif // AWSLC_WINDOWS_7_COMPAT || (WINAPI_PARTITION_APP && !WINAPI_PARTITION_DESKTOP)
 
 OPENSSL_MSVC_PRAGMA(warning(pop))
 
-#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP) && \
-    !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+#if defined(AWSLC_WINDOWS_7_COMPAT) || \
+    (WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP) && \
+    !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP))
 
 void CRYPTO_sysrand(uint8_t *out, size_t requested) {
   while (requested > 0) {
@@ -83,5 +90,5 @@ void CRYPTO_sysrand(uint8_t *out, size_t requested) {
   }
 }
 
-#endif  // WINAPI_PARTITION_APP && !WINAPI_PARTITION_DESKTOP
+#endif  // AWSLC_WINDOWS_7_COMPAT || (WINAPI_PARTITION_APP && !WINAPI_PARTITION_DESKTOP)
 #endif  // OPENSSL_RAND_WINDOWS