backends: ppc use define instead of const for size of dwarf_regs array.

backends: ppc use define instead of const for size of dwarf_regs array.

The size of the dwarf_regs is a constant, but when building without
optimizations the compiler doesn't see that and will warn that it
cannot proof the stack size is bounded. Use a define instead of a
const, so the compiler will use a constant expression everywhere.

Signed-off-by: Mark Wielaard <mark@klomp.org>