crt: Make log2() return exact results for powers of two

These routines are used on msvcrt.dll on arm, which only provides
log(). For exact powers of two, produce an exact result instead
of one that only is numerically close.

Defer to the normal implementation for anything other than the
simple cases (negative values, denormals, infinities, etc).

Signed-off-by: Martin Storsjö <martin@martin.st>