Support for the GCC `__builtin_bswap*` intrinsics is unimplemented for the incremental smt2 decision procedure

[thomasspriggs]

Gcc defines the following builtin / intrinsic functions -

uint16_t __builtin_bswap16(uint16_t);
uint32_t __builtin_bswap32(uint32_t);
uint64_t __builtin_bswap64(uint64_t);

They reverse the order of the bytes which the integers are composed from. These functions arrive at the decision procedure as bswap_exprt or byte swap expressions. This functionality is covered by the regression test cbmc/gcc_bswap1/test.desc.