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/binemit/mod.rs

@@ -59,6 +59,11 @@ pub enum Reloc {
     /// Set the add immediate field to the low 12 bits of the final address. Does not check for overflow.
     /// This is equivalent to `R_AARCH64_TLSGD_ADD_LO12_NC` in the [aaelf64](https://github.com/ARM-software/abi-aa/blob/2bcab1e3b22d55170c563c3c7940134089176746/aaelf64/aaelf64.rst#relocations-for-thread-local-storage)
     Aarch64TlsGdAddLo12Nc,
+
+    /// s390x TLS GD64 - 64-bit offset of tls_index for GD symbol in GOT
+    S390xTlsGd64,
+    /// s390x TLS GDCall - marker to enable optimization of TLS calls
+    S390xTlsGdCall,
 }
 
 impl fmt::Display for Reloc {
@@ -79,6 +84,8 @@ impl fmt::Display for Reloc {
             Self::MachOX86_64Tlv => write!(f, "MachOX86_64Tlv"),
             Self::Aarch64TlsGdAdrPage21 => write!(f, "Aarch64TlsGdAdrPage21"),
             Self::Aarch64TlsGdAddLo12Nc => write!(f, "Aarch64TlsGdAddLo12Nc"),
+            Self::S390xTlsGd64 => write!(f, "TlsGd64"),
+            Self::S390xTlsGdCall => write!(f, "TlsGdCall"),
         }
     }
 }