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/module/src/lib.rs

@@ -78,5 +78,6 @@ pub fn default_libcall_names() -> Box<dyn Fn(ir::LibCall) -> String + Send + Syn
         ir::LibCall::Memcmp => "memcmp".to_owned(),
 
         ir::LibCall::ElfTlsGetAddr => "__tls_get_addr".to_owned(),
+        ir::LibCall::ElfTlsGetOffset => "__tls_get_offset".to_owned(),
     })
 }