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/isa/s390x/lower.rs

@@ -189,6 +189,7 @@ impl LowerBackend for S390xBackend {
             | Opcode::StackAddr
             | Opcode::FuncAddr
             | Opcode::SymbolValue
+            | Opcode::TlsValue
             | Opcode::GetFramePointer
             | Opcode::GetStackPointer
             | Opcode::GetReturnAddress => {
@@ -200,7 +201,6 @@ impl LowerBackend for S390xBackend {
             }
 
             Opcode::ConstAddr
-            | Opcode::TlsValue
             | Opcode::GetPinnedReg
             | Opcode::SetPinnedReg
             | Opcode::Vsplit