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/known_symbol.rs

@@ -0,0 +1,42 @@
+use core::fmt;
+use core::str::FromStr;
+#[cfg(feature = "enable-serde")]
+use serde::{Deserialize, Serialize};
+
+/// A well-known symbol.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
+pub enum KnownSymbol {
+    /// ELF well-known linker symbol _GLOBAL_OFFSET_TABLE_
+    ElfGlobalOffsetTable,
+}
+
+impl fmt::Display for KnownSymbol {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Debug::fmt(self, f)
+    }
+}
+
+impl FromStr for KnownSymbol {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "ElfGlobalOffsetTable" => Ok(Self::ElfGlobalOffsetTable),
+            _ => Err(()),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parsing() {
+        assert_eq!(
+            "ElfGlobalOffsetTable".parse(),
+            Ok(KnownSymbol::ElfGlobalOffsetTable)
+        );
+    }
+}