Provide logb implementation.

logb was taken from upstream bionic's msun folder (as of today). The
only local change is the definition of __weak_reference in s_logb.c.

Fixes the 5 libc++ tests that failed to build with "error: undefined
reference to 'logbl'" in these folders:
numerics/complex.number/complex.member.ops
numerics/complex.number/complex.ops
numerics/complex.number/complex.transcendentals

Change-Id: I112b824107630f3c0861a95ddf4a4a7c11b01999
Merge pull request #14 from awong-dev/logb

Provide logb implementation.