[ELF] Always set _GLOBAL_OFFSET_TABLE_ to the beginning of .got

Previously, we set the address of `_GLOBAL_OFFSET_TABLE_` to `.got.plt`
on i386 and x86-64 and to `.got` on other targets. But it looks like
this special treament of x86 isn't necessary. The x86-64 psABI said
that the symbol can even be in the middle of `.got` (*1). If `.got.plt`
is missing, GNU linker set it to `.got`.

This commit unconditionally set the symbol value to `.got`.

(*1) x86-64 psABI 1.0 p.77: "The symbol _GLOBAL_OFFSET_TABLE_ may
reside in the middle of the .got section, allowing both negative and
non-negative offsets into the array of addresses."

elf/passes.cc

@@ -1323,12 +1323,7 @@ void fix_synthetic_symbols(Context<E> &ctx) {
   start(ctx._DYNAMIC, ctx.dynamic);
 
   // _GLOBAL_OFFSET_TABLE_
-  if (E::e_machine == EM_X86_64 || E::e_machine == EM_386)
-    start(ctx._GLOBAL_OFFSET_TABLE_, ctx.gotplt);
-  else if (E::e_machine == EM_AARCH64 || E::e_machine == EM_RISCV)
-    start(ctx._GLOBAL_OFFSET_TABLE_, ctx.got);
-  else
-    unreachable();
+  start(ctx._GLOBAL_OFFSET_TABLE_, ctx.got);
 
   // __GNU_EH_FRAME_HDR
   start(ctx.__GNU_EH_FRAME_HDR, ctx.eh_frame_hdr);