powerpc: Ensure .mem(init|exit).text are within _stext/_etext

In our linker script we open code the list of text sections, because we
need to include the __ftr_alt sections, which are arch-specific.

This means we can't use TEXT_TEXT as defined in vmlinux.lds.h, and so we
don't have the MEM_KEEP() logic for memory hotplug sections.

If we build the kernel with the gold linker, and with CONFIG_MEMORY_HOTPLUG=y,
we see that functions marked __meminit can end up outside of the
_stext/_etext range, and also outside of _sinittext/_einittext, eg:

    c000000000000000 T _stext
    c0000000009e0000 A _etext
    c0000000009e3f18 T hash__vmemmap_create_mapping
    c000000000ca0000 T _sinittext
    c000000000d00844 T _einittext

This causes them to not be recognised as text by is_kernel_text(), and
prevents them being patched by jump_label (and presumably ftrace/kprobes
etc.).

Fix it by adding MEM_KEEP() directives, mirroring what TEXT_TEXT does.

This isn't a problem when CONFIG_MEMORY_HOTPLUG=n, because we use the
standard INIT_TEXT_SECTION() and EXIT_TEXT macros from vmlinux.lds.h.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>