x86/boot: Explicitly list .{sym,shstr,str}tab in build32.lds.S

Currently, building with LLVM's LLD fails:

    ld -melf_i386_fbsd  --orphan-handling=error -N -T ...
    ld: error: <internal>:(.symtab) is being placed in '.symtab'
    ld: error: <internal>:(.shstrtab) is being placed in '.shstrtab'
    ld: error: <internal>:(.strtab) is being placed in '.strtab'
    gmake[11]: *** [arch/x86/boot/Makefile:69: arch/x86/boot/built-in-32.base.bin] Error 1

This is a consequence of --orphan-handling, and it appears that Binutils
doesn't diagnose some orphaned sections even explicitly asked to do so.

List the sections explicitly.

Fixes: aa9045e771 ('x86/boot: Rework how 32bit C is linked/included for early boot')
Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/boot/build32.lds.S

@@ -66,6 +66,15 @@ SECTIONS
        *(.comment.*)
        *(.note.*)
   }
+  .shstrtab : {
+       *(.shstrtab)
+  }
+  .strtab : {
+       *(.strtab)
+  }
+  .symtab : {
+       *(.symtab)
+  }
   /* Dynamic linkage sections.  Collected simply so we can check they're empty. */
   .got : {
         *(.got)