(perl #133510) use quadmath versions of log10, ldexp and signbit

(perl #133510) use quadmath versions of log10, ldexp and signbit

With -Dquadmath C++ builds, the calls to log10() and ldexp() would
cause ambiguous overloaded function errors, since all of log10(float),
log10(double) and log10(long double) were canidates for a
log10(__float128) call.  Similarly for ldexp().

signbit() had a different problem, two of the tests in ext/POSIX/t/math.t
failed with the default signbit() macro, presumably because the
__float128 was being converted to a long double, since the macro in
math.h didn't special case for __float128.