Default relax_elf_relocations to true

This option tells LLVM to emit relaxable relocation types
R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX/R_386_GOT32X in applicable cases. True
matches Clang's CMake default since 2020-08 [1] and latest LLVM default[2].

This also works around a GNU ld<2.41 issue[3] when using
general-dynamic/local-dynamic TLS models in `-Z plt=no` mode with latest LLVM.

[1]: c41a18cf61
[2]: 2aedfdd9b8
[3]: https://sourceware.org/bugzilla/show_bug.cgi?id=24784

compiler/rustc_target/src/spec/mod.rs

@@ -2216,7 +2216,7 @@ impl Default for TargetOptions {
             mcount: "mcount".into(),
             llvm_mcount_intrinsic: None,
             llvm_abiname: "".into(),
-            relax_elf_relocations: false,
+            relax_elf_relocations: true,
             llvm_args: cvs![],
             use_ctors_section: false,
             eh_frame_header: true,