powerpc: EXPORT_SYMBOL(.TOC.)

For the ELFv2 ABI, powerpc introduces a magic symbol ".TOC.".  depmod
then complains that this doesn't resolve (so does modpost, but we could
easily fix that).  To export this, we need to use asm.

modpost and depmod both strip "." from symbols for the old PPC64 ELFv1
ABI, so we actually export a "TOC.".

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>