Rollup merge of rust-lang#62698 - jethrogb:jb/sgx-llvm-target, r=alexcrichton

SGX target: don't pretend to be GNU/Linux to LLVM

See rust-lang#62592 & rust-lang/compiler-builtins#303

r? @alexcrichton

Author: Mark-Simulacrum

src/librustc_target/spec/x86_64_fortanix_unknown_sgx.rs

@@ -65,7 +65,7 @@ pub fn target() -> Result<Target, String> {
         ..Default::default()
     };
     Ok(Target {
-        llvm_target: "x86_64-unknown-linux-gnu".into(),
+        llvm_target: "x86_64-elf".into(),
         target_endian: "little".into(),
         target_pointer_width: "64".into(),
         target_c_int_width: "32".into(),