Add ELF TLS support in new x64 backend.

This follows the implementation in the legacy x86 backend, including
hardcoded sequence that is compatible with what the linker expects. We
could potentially do better here, but it is likely not necessary.

Thanks to @bjorn3 for a bugfix to an earlier version of this.

cranelift/codegen/src/isa/x64/inst/emit.rs

@@ -1,5 +1,6 @@
 use crate::binemit::{Addend, Reloc};
 use crate::ir::immediates::{Ieee32, Ieee64};
+use crate::ir::LibCall;
 use crate::ir::TrapCode;
 use crate::isa::x64::inst::args::*;
 use crate::isa::x64::inst::*;
@@ -2988,6 +2989,33 @@ pub(crate) fn emit(
         Inst::EpiloguePlaceholder => {
             // Generate no code.
         }
+
+        Inst::ElfTlsGetAddr { ref symbol } => {
+            // N.B.: Must be exactly this byte sequence; the linker requires it,
+            // because it must know how to rewrite the bytes.
+
+            // data16 lea gv@tlsgd(%rip),%rdi
+            sink.put1(0x66); // data16
+            sink.put1(0b01001000); // REX.W
+            sink.put1(0x8d); // LEA
+            sink.put1(0x3d); // ModRM byte
+            emit_reloc(sink, state, Reloc::ElfX86_64TlsGd, symbol, -4);
+            sink.put4(0); // offset
+
+            // data16 data16 callq __tls_get_addr-4
+            sink.put1(0x66); // data16
+            sink.put1(0x66); // data16
+            sink.put1(0b01001000); // REX.W
+            sink.put1(0xe8); // CALL
+            emit_reloc(
+                sink,
+                state,
+                Reloc::X86CallPLTRel4,
+                &ExternalName::LibCall(LibCall::ElfTlsGetAddr),
+                -4,
+            );
+            sink.put4(0); // offset
+        }
     }
 
     state.clear_post_insn();