lib/libffi: Update libffi to 3.4.6.

Libffi is updated to the latest stable version (v3.4.6) as the version
of libffi that was linked by the repository predates the introduction of
RISC-V processors (both 32 and 64 bits).

This is necessary as the packaged version of libffi used by the CI for
RISC-V 64 turned out to simply not work correctly, whilst a source-built
version of libffi worked just fine.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

.gitmodules

@@ -3,7 +3,7 @@
 	url = https://github.com/micropython/axtls.git
 [submodule "lib/libffi"]
 	path = lib/libffi
-	url = https://github.com/atgreen/libffi
+	url = https://github.com/libffi/libffi
 [submodule "lib/lwip"]
 	path = lib/lwip
 	url = https://github.com/lwip-tcpip/lwip.git

ports/unix/Makefile

@@ -168,7 +168,7 @@ ifeq ($(MICROPY_STANDALONE),1)
 # Build libffi from source.
 GIT_SUBMODULES += lib/libffi
 DEPLIBS += libffi
-LIBFFI_CFLAGS := -I$(shell ls -1d $(BUILD)/lib/libffi/out/lib/libffi-*/include)
+LIBFFI_CFLAGS := -I$(shell ls -1d $(BUILD)/lib/libffi/include)
  ifeq ($(MICROPY_FORCE_32BIT),1)
   LIBFFI_LDFLAGS = $(BUILD)/lib/libffi/out/lib32/libffi.a
  else