Object file support for s390x (#2872)

Add support for s390x binary format object files.  In particular,
add support for s390x ELF relocation types (currently only
S390xPCRel32Dbl).

cranelift/codegen/src/binemit/mod.rs

@@ -60,6 +60,8 @@ pub enum Reloc {
     Arm64Call,
     /// RISC-V call target
     RiscvCall,
+    /// s390x PC-relative 4-byte offset
+    S390xPCRel32Dbl,
 
     /// Elf x86_64 32 bit signed PC relative offset to two GOT entries for GD symbol.
     ElfX86_64TlsGd,
@@ -75,6 +77,7 @@ impl fmt::Display for Reloc {
         match *self {
             Self::Abs4 => write!(f, "Abs4"),
             Self::Abs8 => write!(f, "Abs8"),
+            Self::S390xPCRel32Dbl => write!(f, "PCRel32Dbl"),
             Self::X86PCRel4 => write!(f, "PCRel4"),
             Self::X86PCRelRodata4 => write!(f, "PCRelRodata4"),
             Self::X86CallPCRel4 => write!(f, "CallPCRel4"),