powerpc: Put allocated ELF notes in read-only data segment

This changes the powerpc linker script to use the asm-generic NOTES macro so
that ELF note sections with SHF_ALLOC set are linked into the kernel image
along with other read-only data.  The PT_NOTE also points to their location.

This paves the way for putting useful build-time information into ELF notes
that can be found easily later in a kernel memory dump.

Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>