ext: Update softfloat to 3d full version

* Add all softfloat source files without any change.
* Remove useless file softfloat.mk.in, since gem5 use Scons.
* Add `use_fast_int64` in SConscript to distinguish src of two strategies for data
  larger than 64 bits.
  * The SoftFloat library uses two strategies to handle data larger than 64bit. One is
    spliting data into `fast_int64`, and the other is using pointer. Two strategies
    are distinguished by macro `SOFTFLOAT_FAST_INT64`. But not all "*.c" files are
    guarded by this macro, which leads to including useless files in compiling progress
    and compiling error. `use_fast_int64` used in SConscript can exclude unnecessary
    files.

Change-Id: I7cec10412c00a35c247299cd92d83cdee9066410
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/66552
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>

ext/softfloat/f16_sub.c

@@ -36,9 +36,8 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <stdbool.h>
 #include <stdint.h>
-
-#include "internals.h"
 #include "platform.h"
+#include "internals.h"
 #include "softfloat.h"
 
 float16_t f16_sub( float16_t a, float16_t b )