test/codegen: add UEFI cross-compilation test

Add a new codegen test which cross-compiles a UEFI program and verifies
that an entry-point is emitted.

The UEFI code-generation has broken in nightly before, see for instance:

    commit c1b2fd2
    Merge: faccb0f 6e77729
    Author: Dylan DPC <[email protected]>
    Date:   Tue May 5 01:49:44 2020 +0200

        Rollup merge of rust-lang#71881 - IsaacWoods:master, r=petrochenkov

        Correctly handle UEFI targets as Windows-like when emitting sections for LLVM bitcode

This commit adds a very simple UEFI program and verifies that
cross-compilation with the built-in `x86_64-unknown-uefi` target works.
No runtime testing, nor any elaborate code-validation is done. This
simply serves as base test that verifies compilation works and an entry
point is emitted.

Usually, you would use `libcore`, but in this test we do not want to
trigger re-compilation of `libcore` for `x86_64-unknown-uefi`. Hence, we
pull in just enough of `libcore` to make the test compile.

This UEFI program is fully functional and returns success (i.e., 0). You
can run it from your UEFI BootManager or from EfiShell.

The imported UEFI definitions are explicitly kept simple. For more
elaborate use-cases you would want the full UEFI specification (see the
`r_efi` crate for a standalone import of the base UEFI specification).

Author: dvdhrm

src/test/codegen/uefi-cross-compile.rs

@@ -0,0 +1,30 @@
+// Checks whether UEFI targets cross-compile successfully.
+//
+// This test contains a simple UEFI program that simply exits with return code
+// 0. It can be easily run from the UEFI shell (but any other UEFI environment
+// works as well). This program is not run as part of the test. The test merely
+// verifies the cross-compilation does not fail and an entry-point is emitted.
+//
+// The imported definitions from the UEFI specification are intentionally left
+// incomplete. Only the bits that are actually used by this test are defined.
+
+// min-llvm-version 9.0
+
+// compile-flags: --target x86_64-unknown-uefi
+
+#![feature(abi_efiapi, lang_items, no_core)]
+#![no_core]
+#![no_main]
+
+#[lang = "sized"]
+pub trait Sized {}
+#[lang = "freeze"]
+pub trait Freeze {}
+#[lang = "copy"]
+pub trait Copy {}
+
+// CHECK: define win64cc i64 @efi_main{{.*}}
+#[export_name = "efi_main"]
+pub extern "efiapi" fn main(_h: *mut usize, _st: *mut usize) -> usize {
+    return 0;
+}