Enable Windows MinGW support (#1000)

Allow compilation on Windows MinGW, see build_wamr.md for more details.

Note that WASI and some other smallish details are still not supported, but
we have a starting point. See more discussion at #993

core/shared/platform/windows/win_time.c

@@ -9,7 +9,12 @@ uint64
 os_time_get_boot_microsecond()
 {
     struct timespec ts;
+#if defined(__MINGW32__)
+    // https://www.mail-archive.com/mingw-w64-public@lists.sourceforge.net/msg18361.html
+    clock_gettime(CLOCK_REALTIME, &ts);
+#else
     timespec_get(&ts, TIME_UTC);
+#endif
 
     return ((uint64)ts.tv_sec) * 1000 * 1000 + ((uint64)ts.tv_nsec) / 1000;
 }

core/shared/utils/uncommon/bh_read_file.c

@@ -9,6 +9,11 @@
 #endif
 
 #if defined(_WIN32) || defined(_WIN32_)
+
+#if defined(__MINGW32__) && !defined(_SH_DENYNO)
+#define _SH_DENYNO 0x40
+#endif
+
 char *
 bh_read_file_to_buffer(const char *filename, uint32 *ret_size)
 {