C++ API: Reintroduce zero-value mkBitVector method (#2770)

C++ API: Reintroduce zero-value mkBitVector method (#2770)

PR #2764 removed `Solver::mkBitVector(uint32_t)` (returns a bit-vector
of a given size with value zero), which made the build fail when SymFPU
was enabled because solver_black used it for SymFPU-enabled builds. This
commit simply adds a zero default argument to `mkBitVector(uint32_t,
uint64_t)` to allow users to create zero-valued bit-vectors without
explicitly specifying the value again. Additionally, the commit replaces
the use of the `CVC4_USE_SYMFPU` macro by a call to
`Configuration::isBuiltWithSymFPU()`, making sure that we can catch
compile-time errors regardless of configuration. Finally,
`Solver::mkConst(Kind, uint32_t, uint32_t, Term)` now checks whether
CVC4 has been compiled with SymFPU when creating a `CONST_FLOATINGPOINT`
and throws an exception otherwise (solver_black has been updated
correspondingly).