Implement -Z relax-elf-relocations=yes|no

This lets rustc users tweak whether the linker should relax ELF relocations,
namely whether it should emit R_X86_64_GOTPCRELX relocations instead of
R_X86_64_GOTPCREL, as the former is allowed by the ABI to be further
optimised. The default value is whatever the target defines.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -156,7 +156,11 @@ pub fn target_machine_factory(
     let emit_stack_size_section = sess.opts.debugging_opts.emit_stack_sizes;
 
     let asm_comments = sess.asm_comments();
-    let relax_elf_relocations = sess.target.options.relax_elf_relocations;
+    let relax_elf_relocations = sess
+        .opts
+        .debugging_opts
+        .relax_elf_relocations
+        .unwrap_or(sess.target.options.relax_elf_relocations);
 
     let use_init_array = !sess
         .opts