aarch64: Implement TLS ELF GD Relocations

Implement the `TlsValue` opcode in the aarch64 backend for ELF_GD.

This is a little bit unusual as the default TLS mechanism for aarch64 is TLS Descriptors in other compilers.
However currently we only recognize elf_gd so lets start with that as a TLS implementation.

cranelift/codegen/src/binemit/mod.rs

@@ -68,6 +68,16 @@ pub enum Reloc {
 
     /// Mach-O x86_64 32 bit signed PC relative offset to a `__thread_vars` entry.
     MachOX86_64Tlv,
+
+    /// AArch64 TLS GD
+    /// Set an ADRP immediate field to the top 21 bits of the final address. Checks for overflow.
+    /// This is equivalent to `R_AARCH64_TLSGD_ADR_PAGE21` in the [aaelf64](https://github.com/ARM-software/abi-aa/blob/2bcab1e3b22d55170c563c3c7940134089176746/aaelf64/aaelf64.rst#relocations-for-thread-local-storage)
+    Aarch64TlsGdAdrPage21,
+
+    /// AArch64 TLS GD
+    /// 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,
 }
 
 impl fmt::Display for Reloc {
@@ -87,6 +97,8 @@ impl fmt::Display for Reloc {
 
             Self::ElfX86_64TlsGd => write!(f, "ElfX86_64TlsGd"),
             Self::MachOX86_64Tlv => write!(f, "MachOX86_64Tlv"),
+            Self::Aarch64TlsGdAdrPage21 => write!(f, "Aarch64TlsGdAdrPage21"),
+            Self::Aarch64TlsGdAddLo12Nc => write!(f, "Aarch64TlsGdAddLo12Nc"),
         }
     }
 }