misc/bb: fix building on non-x86

regparm is strictly i386, but clang allows it on amd64.

Building on other architectures fails with:
In file included from zoom.c:56:
In file included from ./zoom.h:30:
./formulas.h:53:63: error: 'regparm' is not valid on this platform

int (*calculate) (number_t, number_t, number_t, number_t) REGISTERS(3);
                                                          ^         ~

./config.h:42:38: note: expanded from macro 'REGISTERS'

GCC builts in just fine.