Implement hypotl in terms of hypot.

hypotl() returns invalid results, for example hypotl(3, 4) returns
0x1.3ffffffffffffp+2 instead of 0x1.4p+2. hypot() doesn't have this
problem, and double and long double are both 64 bit on Android.

Fixes 1 failure in libc++'s numerics/complex.number/complex.value.ops

Change-Id: Ia7e54cd22af09ab9246bb78d42b882336775ea4d
Merge pull request #15 from awong-dev/hypotl

Implement hypotl in terms of hypot.