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/ir/libcall.rs

@@ -66,6 +66,8 @@ pub enum LibCall {
 
     /// Elf __tls_get_addr
     ElfTlsGetAddr,
+    /// Elf __tls_get_offset
+    ElfTlsGetOffset,
     // When adding a new variant make sure to add it to `all_libcalls` too.
 }
 
@@ -104,6 +106,7 @@ impl FromStr for LibCall {
             "Memcmp" => Ok(Self::Memcmp),
 
             "ElfTlsGetAddr" => Ok(Self::ElfTlsGetAddr),
+            "ElfTlsGetOffset" => Ok(Self::ElfTlsGetOffset),
             _ => Err(()),
         }
     }
@@ -173,6 +176,7 @@ impl LibCall {
             Memmove,
             Memcmp,
             ElfTlsGetAddr,
+            ElfTlsGetOffset,
         ]
     }
 
@@ -214,7 +218,8 @@ impl LibCall {
             | LibCall::Memset
             | LibCall::Memmove
             | LibCall::Memcmp
-            | LibCall::ElfTlsGetAddr => unimplemented!(),
+            | LibCall::ElfTlsGetAddr
+            | LibCall::ElfTlsGetOffset => unimplemented!(),
         }
 
         sig