[libc] fix stdbit include test when not all entrypoints are available (#80323)

The intent of the test is to check that: 1. The type generic macros are
defined. 2. Those macros dispatch to the correct underlying function.

The issue is that when new functionality is added to our stdbit.h without
rolling out the new entrypoint to all targets, this test breaks because our
generated stdbit.h will not contain declarations for the underlying function.
In that case, we should just declare the underlying functions first before
including our generated stdbit.h which just contains declarations. A definition
is a declaration, but redeclarations must match, hence the additions of
noexcept and extern "C".