x86/build32: Discard all orphaned sections

Linkers may put orphaned sections ahead of .text, which breaks the calling
requirements.  A concrete example is Ubuntu's GCC-9 default of enabling
-fcf-protection which causes us to try and execute .note.gnu.properties during
Xen's boot.

Put .got.plt in its own section as it specifically needs preserving from the
linkers point of view, and discard everything else.  This will hopefully be
more robust to other unexpected toolchain properties.

Fixes boot from an Ubuntu build of Xen.

Reported-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/boot/build32.lds

@@ -31,7 +31,7 @@ SECTIONS
         *(.bss.*)
   }
 
-  /DISCARD/ : {
+  .got.plt : {
         /*
          * PIC/PIE executable contains .got.plt section even if it is not linked
          * with dynamic libraries. In such case it is just placeholder for
@@ -47,6 +47,14 @@ SECTIONS
          *
          * Please check build32.mk for more details.
          */
-        /* *(.got.plt) */
+        *(.got.plt)
+  }
+
+  /DISCARD/ : {
+        /*
+         * Discard everything else, to prevent linkers from putting
+         * orphaned sections ahead of .text, which needs to be first.
+         */
+        *(*)
   }
 }