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/mod.rs

@@ -15,6 +15,7 @@ mod heap;
 pub mod immediates;
 pub mod instructions;
 pub mod jumptable;
+pub(crate) mod known_symbol;
 pub mod layout;
 pub(crate) mod libcall;
 mod memflags;
@@ -50,6 +51,7 @@ pub use crate::ir::instructions::{
     InstructionData, Opcode, ValueList, ValueListPool, VariableArgs,
 };
 pub use crate::ir::jumptable::JumpTableData;
+pub use crate::ir::known_symbol::KnownSymbol;
 pub use crate::ir::layout::Layout;
 pub use crate::ir::libcall::{get_probestack_funcref, LibCall};
 pub use crate::ir::memflags::{Endianness, MemFlags};