s390x: Implement tls_value (#4616)

Implement the tls_value for s390 in the ELF general-dynamic mode.

Notable differences to the x86_64 implementation are:
- We use a __tls_get_offset libcall instead of __tls_get_addr.
- The current thread pointer (stored in a pair of access registers)
  needs to be added to the result of __tls_get_offset.
- __tls_get_offset has a variant ABI that requires the address of
  the GOT (global offset table) is passed in %r12.

This means we need a new libcall entries for __tls_get_offset.
In addition, we also need a way to access _GLOBAL_OFFSET_TABLE_.
The latter is a "magic" symbol with a well-known name defined
by the ABI and recognized by the linker.  This patch introduces
a new ExternalName::KnownSymbol variant to support such names
(originally due to @afonso360).

We also need to emit a relocation on a symbol placed in a
constant pool, as well as an extra relocation on the call
to __tls_get_offset required for TLS linker optimization.

Needed by the cg_clif frontend.

cranelift/codegen/src/machinst/isle.rs

@@ -15,6 +15,7 @@ pub use crate::isa::unwind::UnwindInst;
 pub use crate::machinst::{
     ABIArg, ABIArgSlot, ABISig, InputSourceInst, RealReg, Reg, RelocDistance, Writable,
 };
+pub use crate::settings::TlsModel;
 
 pub type Unit = ();
 pub type ValueSlice = (ValueList, usize);
@@ -633,6 +634,15 @@ macro_rules! isle_prelude_methods {
             }
         }
 
+        #[inline]
+        fn tls_model_is_elf_gd(&mut self) -> Option<()> {
+            if self.flags.tls_model() == TlsModel::ElfGd {
+                Some(())
+            } else {
+                None
+            }
+        }
+
         #[inline]
         fn func_ref_data(&mut self, func_ref: FuncRef) -> (SigRef, ExternalName, RelocDistance) {
             let funcdata = &self.lower_ctx.dfg().ext_funcs[func_ref];