util: Implement os_get_available_system_memory on Win32

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18001>

src/util/os_misc.c

@@ -317,6 +317,14 @@ os_get_available_system_memory(uint64_t *size)
 
    *size = MIN2(mem_available, rl.rlim_cur);
    return true;
+#elif DETECT_OS_WINDOWS
+   MEMORYSTATUSEX status;
+   BOOL ret;
+
+   status.dwLength = sizeof(status);
+   ret = GlobalMemoryStatusEx(&status);
+   *size = status.ullAvailPhys;
+   return (ret == TRUE);
 #else
    return false;
 #endif