usr.bin/bc: fix build issue of version 6.6.0 on MIPS

The update removed MIPS and POWERPC64 from the list of architectures
that cannot use LTO to build this software.

Restore the previous exception list and do not use LTO on MIPS,
MIPS64, POWERPC64, and RISCV64. This is necessary due to differences
in compiler support for LTO in -CURRENT vs. 13-STABLE.