powerpc: Fix COFF zImage booting on old powermacs

[Michael Ellerman]

On Mon, 2018-11-26 at 22:01:54 UTC, Paul Mackerras wrote:
> Commit 6975a783d7b4 ("powerpc/boot: Allow building the zImage wrapper
> as a relocatable ET_DYN", 2011-04-12) changed the procedure descriptor
> at the start of crt0.S to have a hard-coded start address of 0x500000
> rather than a reference to _zimage_start, presumably because having
> a reference to a symbol introduced a relocation which is awkward to
> handle in a position-independent executable.  Unfortunately, what is
> at 0x500000 in the COFF image is not the first instruction, but the
> procedure descriptor itself, that is, a word containing 0x500000,
> which is not a valid instruction.  Hence, booting a COFF zImage
> results in a "DEFAULT CATCH!, code=FFF00700" message from Open
> Firmware.
> 
> This fixes the problem by (a) putting the procedure descriptor in the
> data section and (b) adding a branch to _zimage_start as the first
> instruction in the program.
> 
> Fixes: 6975a783d7b4 ("powerpc/boot: Allow building the zImage wrapper as a relocatable ET_DYN")
> Signed-off-by: Paul Mackerras <paulus at ozlabs.org>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/5564597d51c8ff5b88d95c76255e18

cheers