kbuild: move -fno-dwarf2-cfi-asm to powerpc only

kbuild: move -fno-dwarf2-cfi-asm to powerpc only

Better dwarf2 unwind information is a good thing, it allows better
debugging with kgdb and crash and helps systemtap.

Commit 003086497f07f7f1e67c0c295e261740f822b377 ("Build with
-fno-dwarf2-cfi-asm") disabled some CFI information globally to work
around a module loader bug on powerpc.

But this disables the better unwind tables for all architectures, not just
powerpc.  Move the workaround to powerpc and also add a suitable comment
that's it really a workaround.

This improves dwarf2 unwind tables on x86 at least.

Signed-off-by: Andi Kleen <[email protected]>
Cc: Kyle McMartin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Benjamin Herrenschmidt <[email protected]>
Signed-off-by: Michal Marek <[email protected]>