Move {mul,sqr}_add_c{,2} macros from bn_asm.c to bn_local.h.

Move {mul,sqr}_add_c{,2} macros from bn_asm.c to bn_local.h.

These depend on other macros that are in already in bn_local.h and this
makes them available to other source files. A lot more clean up will be
needed in the future.

Of course x86_64-gcc.c makes use of the same macro names - sprinkle some
undef in there for the time being.

ok tb@