powerpc: Fix HMIs on big-endian with CONFIG_RELOCATABLE=y

HMIs will crash the kernel due to

	BRANCH_LINK_TO_FAR(hmi_exception_realmode)

Calling into the OPD instead of the actual code.

Fixes: 2337d207

 ("powerpc/64: CONFIG_RELOCATABLE support for hmi interrupts")
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[mpe: Use DOTSYM() rather than #ifdef]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>