[PowerPC] Fix build-id note on powerpc64 kernel

Due to the ordering of the powerpc64 linker script, we were discarding
all notes before emitting .note.gnu.build-id. This had the effect of
generating an empty build id section and breaking the kern.build_id
sysctl added in r348611.

powerpc and powerpcspe are uneffected.

PR:		246430
MFC after:	3 days
Sponsored by:	Tag1 Consulting, Inc.

sys/conf/ldscript.powerpc64

@@ -37,9 +37,6 @@ SECTIONS
   /* Do not emit PT_INTERP section, which confuses some loaders (kexec-lite) */
   /DISCARD/   : { *(.interp)	} 
 
-  /* Also delete notes */
-  /DISCARD/   : { *(.note.*)	} 
-
   .hash          : { *(.hash)		}
   .dynsym        : { *(.dynsym)		}
   .dynstr        : { *(.dynstr)		}
@@ -51,6 +48,10 @@ SECTIONS
     *(.note.gnu.build-id)
     PROVIDE (__build_id_end = .);
   }
+
+  /* Do not emit any additional notes. */
+  /DISCARD/   : { *(.note.*)	} 
+
   .rela.text     :
     { *(.rela.text) *(.rela.gnu.linkonce.t*) }
   .rela.data     :