tests/codegen-llvm: Make rust-abi-arch-specific-adjustment portable

This test currently only runs on RISC-V and loongarch hosts, but assumes
that the host target is the -gnu target. By using minicore, we can
run this test on all host targets, regardless of architecture, as long
as the LLVM components are built.
This also fixes this test on musl hosts of these architectures.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

tests/codegen-llvm/rust-abi-arch-specific-adjustment.rs

@@ -1,15 +1,19 @@
+//@ add-core-stubs
 //@ compile-flags: -Copt-level=3 -C no-prepopulate-passes
 //@ revisions: riscv64 loongarch64
 
-//@[riscv64] only-riscv64
 //@[riscv64] compile-flags: --target riscv64gc-unknown-linux-gnu
 //@[riscv64] needs-llvm-components: riscv
 
-//@[loongarch64] only-loongarch64
 //@[loongarch64] compile-flags: --target loongarch64-unknown-linux-gnu
 //@[loongarch64] needs-llvm-components: loongarch
 
 #![crate_type = "lib"]
+#![feature(no_core)]
+#![no_core]
+
+extern crate minicore;
+use minicore::*;
 
 #[no_mangle]
 // riscv64:     define noundef i8 @arg_attr_u8(i8 noundef zeroext %x)