RT3992: Make SCT #ifdeffable.

This code does open-coded division on 64-bit quantities and thus when
building with GCC on 32-bit platforms will require functions such as
__umoddi3 and __udivdi3 from libgcc.

In constrained environments such as firmware, those functions may not
be available. So make it possible to compile out SCT support, which in
fact (in the case of UEFI) we don't need anyway.

Signed-off-by: Rich Salz <rsalz@akamai.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>