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/object/src/backend.rs

@@ -121,6 +121,7 @@ pub struct ObjectModule {
     relocs: Vec<SymbolRelocs>,
     libcalls: HashMap<ir::LibCall, SymbolId>,
     libcall_names: Box<dyn Fn(ir::LibCall) -> String + Send + Sync>,
+    known_symbols: HashMap<ir::KnownSymbol, SymbolId>,
     function_alignment: u64,
     per_function_section: bool,
     anon_func_number: u64,
@@ -141,6 +142,7 @@ impl ObjectModule {
             relocs: Vec::new(),
             libcalls: HashMap::new(),
             libcall_names: builder.libcall_names,
+            known_symbols: HashMap::new(),
             function_alignment: builder.function_alignment,
             per_function_section: builder.per_function_section,
             anon_func_number: 0,
@@ -547,6 +549,28 @@ impl ObjectModule {
                     symbol
                 }
             }
+            // These are "magic" names well-known to the linker.
+            // They require special treatment.
+            ir::ExternalName::KnownSymbol(ref known_symbol) => {
+                if let Some(symbol) = self.known_symbols.get(known_symbol) {
+                    *symbol
+                } else {
+                    let symbol = match known_symbol {
+                        ir::KnownSymbol::ElfGlobalOffsetTable => self.object.add_symbol(Symbol {
+                            name: "_GLOBAL_OFFSET_TABLE_".as_bytes().to_vec(),
+                            value: 0,
+                            size: 0,
+                            kind: SymbolKind::Data,
+                            scope: SymbolScope::Unknown,
+                            weak: false,
+                            section: SymbolSection::Undefined,
+                            flags: SymbolFlags::None,
+                        }),
+                    };
+                    self.known_symbols.insert(*known_symbol, symbol);
+                    symbol
+                }
+            }
             _ => panic!("invalid ExternalName {}", name),
         }
     }