Add x86_64-fortanix-unknown-sgx target to libstd and dependencies

The files src/libstd/sys/sgx/*.rs are mostly copied/adapted from
the wasm target.

This also updates the dlmalloc submodule to the very latest version.

Author: Jethro Beekman

.gitmodules

@@ -61,10 +61,12 @@
 	path = src/tools/clang
 	url = https://github.com/rust-lang-nursery/clang.git
 	branch = rust-release-80-v2
-
 [submodule "src/doc/rustc-guide"]
 	path = src/doc/rustc-guide
 	url = https://github.com/rust-lang/rustc-guide.git
 [submodule "src/doc/edition-guide"]
 	path = src/doc/edition-guide
 	url = https://github.com/rust-lang-nursery/edition-guide
+[submodule "src/rust-sgx"]
+	path = src/rust-sgx
+	url = https://github.com/fortanix/rust-sgx

Cargo.lock

@@ -797,6 +797,14 @@ name = "foreign-types-shared"
 version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "fortanix-sgx-abi"
+version = "0.0.0"
+dependencies = [
+ "compiler_builtins 0.0.0",
+ "core 0.0.0",
+]
+
 [[package]]
 name = "fs2"
 version = "0.4.3"
@@ -2773,6 +2781,7 @@ dependencies = [
  "compiler_builtins 0.0.0",
  "core 0.0.0",
  "dlmalloc 0.0.0",
+ "fortanix-sgx-abi 0.0.0",
  "libc 0.0.0",
  "panic_abort 0.0.0",
  "panic_unwind 0.0.0",