aarch64: Implement TLS ELF GD Relocations

Implement the `TlsValue` opcode in the aarch64 backend for ELF_GD.

This is a little bit unusual as the default TLS mechanism for aarch64 is TLS Descriptors in other compilers.
However currently we only recognize elf_gd so lets start with that as a TLS implementation.

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -8,7 +8,7 @@ use crate::ir::{InstructionData, Opcode, TrapCode};
 use crate::isa::aarch64::settings as aarch64_settings;
 use crate::machinst::lower::*;
 use crate::machinst::*;
-use crate::settings::Flags;
+use crate::settings::{Flags, TlsModel};
 use crate::{CodegenError, CodegenResult};
 
 use crate::isa::aarch64::abi::*;
@@ -3489,7 +3489,24 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             });
         }
 
-        Opcode::TlsValue => unimplemented!("tls_value"),
+        Opcode::TlsValue => match flags.tls_model() {
+            TlsModel::ElfGd => {
+                let dst = get_output_reg(ctx, outputs[0]).only_reg().unwrap();
+                let (name, _, _) = ctx.symbol_value(insn).unwrap();
+                let symbol = name.clone();
+                ctx.emit(Inst::ElfTlsGetAddr { symbol });
+
+                let x0 = xreg(0);
+                ctx.emit(Inst::gen_move(dst, x0, I64));
+            }
+            _ => {
+                todo!(
+                    "Unimplemented TLS model in AArch64 backend: {:?}",
+                    flags.tls_model()
+                );
+            }
+        },
+
         Opcode::FcvtLowFromSint => unimplemented!("FcvtLowFromSint"),
         Opcode::FvpromoteLow => unimplemented!("FvpromoteLow"),
         Opcode::Fvdemote => unimplemented!("Fvdemote"),