Engage run-time switch between bn_mul_mont_fpu and bn_mul_mont_int.

possible usage is to pass -Wl,-z,-noexecstack on Linux to ensure the stack
is marked non-executable. Well, -Wa,--noexecstack works fine too...

keep disclaiming narrower than 32-bit support.

have impact on performance, because amount of multiplications does not
increase with this switch, not on sparcv9 that is. On the contrary, it
actually improves performance, because it spares a load of instructions
used to chase carries. Not to mention that BN assembler modules can be
shared more freely between 32- and 64-bit builts.

Submitted by: Douglas Stebila

space, but gives total flexibility [back].

made...