bpo-41100: Support macOS 11 and Apple Silicon (GH-22855)

Co-authored-by:  Lawrence D’Anna <lawrence_danna@apple.com>

* Add support for macOS 11 and Apple Silicon (aka arm64)
   
  As a side effect of this work use the system copy of libffi on macOS, and remove the vendored copy

* Support building on recent versions of macOS while deploying to older versions

  This allows building installers on macOS 11 while still supporting macOS 10.9.
diff --git a/Mac/BuildScript/openssl-mac-arm64.patch b/Mac/BuildScript/openssl-mac-arm64.patch
new file mode 100644
index 0000000..11267fb
--- /dev/null
+++ b/Mac/BuildScript/openssl-mac-arm64.patch
@@ -0,0 +1,41 @@
+diff -ur openssl-1.1.1g-orig/Configurations/10-main.conf openssl-1.1.1g/Configurations/10-main.conf
+--- openssl-1.1.1g-orig/Configurations/10-main.conf	2020-04-21 14:22:39.000000000 +0200
++++ openssl-1.1.1g/Configurations/10-main.conf	2020-07-26 12:21:32.000000000 +0200
+@@ -1557,6 +1557,14 @@
+         bn_ops           => "SIXTY_FOUR_BIT_LONG",
+         perlasm_scheme   => "macosx",
+     },
++    "darwin64-arm64-cc" => {
++        inherit_from     => [ "darwin-common", asm("aarch64_asm") ],
++        CFLAGS           => add("-Wall"),
++        cflags           => add("-arch arm64"),
++        lib_cppflags     => add("-DL_ENDIAN"),
++        bn_ops           => "SIXTY_FOUR_BIT_LONG",
++        perlasm_scheme   => "ios64",
++    },
+ 
+ ##### GNU Hurd
+     "hurd-x86" => {
+diff -ur openssl-1.1.1g-orig/config openssl-1.1.1g/config
+--- openssl-1.1.1g-orig/config	2020-04-21 14:22:39.000000000 +0200
++++ openssl-1.1.1g/config	2020-07-26 12:21:59.000000000 +0200
+@@ -255,6 +255,9 @@
+ 		;;
+ 	    x86_64)
+ 		echo "x86_64-apple-darwin${VERSION}"
++                ;;
++	    arm64)
++		echo "arm64-apple-darwin${VERSION}"
+ 		;;
+ 	    *)
+ 		echo "i686-apple-darwin${VERSION}"
+@@ -497,6 +500,9 @@
+ 	else
+ 	    OUT="darwin64-x86_64-cc"
+ 	fi ;;
++  x86_64-apple-darwin*)
++	OUT="darwin64-arm64-cc"
++        ;;
+   armv6+7-*-iphoneos)
+ 	__CNF_CFLAGS="$__CNF_CFLAGS -arch armv6 -arch armv7"
+ 	__CNF_CXXFLAGS="$__CNF_CXXFLAGS -arch armv6 -arch armv7"